METHOD AND APPARATUS FOR VIRTUAL MACHINE LIVE STORAGE MIGRATION IN HETEROGENEOUS STORAGE ENVIRONMENT

• US 20150234617A1
• Filed: 02/18/2014
• Published: 08/20/2015
• Est. Priority Date: 02/18/2014
• Status: Active Grant

• Current Assignee: University of Florida Research Foundation, Incorporated (State University System of Florida)
• Original Assignee: University of Florida Research Foundation, Incorporated (State University System of Florida)
• Inventors: LI, TAO, ZHOU, RUIJIN
• Granted Patent: US 9,195,401 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G06F 11/14 Error detection or correction of the data by redundancy in operation G06F11/16 takes precedence
• CPC Class Codes: G06F 11/203 using migration
• CPC Class Codes: G06F 11/2064 while ensuring consistency
• CPC Class Codes: G06F 11/2074 Asynchronous techniques
• CPC Class Codes: G06F 2201/815 Virtual middleware or OS functionality using virtual machines to implement generic software techniques for error detection or fault masking G06F11/1484
• CPC Class Codes: G06F 3/0617 in relation to availability
• CPC Class Codes: G06F 3/0619 in relation to data integrity, e.g. data losses, bit errors
• CPC Class Codes: G06F 3/0647 Migration mechanisms
• CPC Class Codes: G06F 3/0665 at area level, e.g. provisioning of virtual or logical volumes
• CPC Class Codes: G06F 3/0685 Hybrid storage combining heterogeneous device types, e.g. hierarchical storage, hybrid arrays

Abstract

Embodiments pertain to live storage migration for virtual machines. Specific embodiments can implement the migration of VM disk images without service interruption to the running workload. Specific embodiments relate to storage migration between different disk arrays. Embodiments of the subject invention relate to a method and apparatus that can enhance the efficiency of virtual machine (VM) live storage migration in heterogeneous storage environments from a multi-dimensional perspective, e.g., user experience, device wearing, and/or manageability. Specific embodiments utilize one or more of the following: adaptive storage migration strategies, or techniques, such as 1) Low Redundancy (LR), which generates a reduced, and preferably the least, amount of redundant writes; 2) Source-based Low Redundancy (SLR), which can help keep a desirable balance between IO performance and write redundancy; and 3) Asynchronous IO Mirroring (AIO), which seeks high, and preferably the highest, IO performance. Specific embodiments adaptively mix one or more of these adaptive storage migration techniques during massive VM live storage migration.

Claims

1. A method of transferring a virtual machine storage, comprising:

copying a virtual machine disk image of a virtual machine from a source storage to a destination storage, wherein the virtual machine disk image has a plurality of data blocks; and

handling virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage,

wherein copying the virtual machine disk image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage is accomplished via (a), (b), or (c), as follows;

(a) copying the plurality of data blocks of the virtual machine disk image from the source storage to the destination storage;

partitioning the virtual machine disk image into a copied region and a to-be-copied region, where the copied region has been copied from the source storage to the destination storage, wherein the to-be-copied region is going to be copied from the source storage to the destination storage;

intercepting the virtual machine IO requests from the virtual machine, where the virtual machine IO requests from the virtual machine comprise write requests to the virtual machine disk image and read requests to the virtual machine disk image, wherein the write requests to the virtual machine disk image comprise write requests to the copied region and write requests to the to-be-copied region, wherein the read requests to the virtual machine disk image comprise read requests to the copied region and read requests to the to-be-copied region;

recording the write requests to the to-be-copied region in a bitmap;

reading data for read requests to the copied region from the destination storage;

wherein for each read request to the to-be-copied region,

(i) checking the bitmap to determine whether a data block corresponding to the each read request to the to-be-copied region is recorded in the bitmap;

(ii) if the data block corresponding to the each read request to the to-be-copied region is recorded in the bitmap, issuing the each read request to the to-be-copied region to the destination storage; and

(iii) if the data block corresponding to the each read request to the to-be-copied region is not recorded in the bitmap, issuing the each read request to the to-be-copied region to the source storage;

wherein for write requests to the virtual machine disk image and read requests to the virtual machine disk image intercepted while data blocks corresponding to such intercepted write requests to the virtual machine disk image and such intercepted read requests to the virtual machine disk image are being copied from the source storage to the destination storage, accomplishing such intercepted write requests to the virtual machine disk image and such intercepted read requests to the virtual machine disk image after the data blocks corresponding to such intercepted write requests to the virtual machine disk image and such intercepted read requests to the virtual machine disk image have been copied from the source storage to the destination storage,

wherein copying the plurality of data blocks of the virtual machine disk image from the source storage to the destination storage comprises;

for each data block of the plurality of data blocks of the virtual machine disk image to be copied from the source storage to the destination storage,

(i) checking the bitmap to determine whether the each data block of the plurality of data blocks of the virtual machine disk image to be copied from the source storage to the destination storage is recorded in the bitmap;

(ii) if the each data block of the plurality of data blocks of the virtual machine disk image to be copied from the source storage to the destination storage is not recorded in the bitmap, copying the each data block of the plurality of data blocks of the virtual machine disk image to be copied from the source storage to the destination storage; and

(iii) if the each data block of the plurality of data blocks of the virtual machine disk image to be copied from the source storage to the destination storage is recorded in the bitmap, skipping copying the each data block of the plurality of data blocks of the virtual machine disk image to be copied from the source storage to the destination storage,

(b) copying the plurality of data blocks of the virtual machine disk image from the source storage to the destination storage,

partitioning the virtual machine disk image into a copied region and a to-be-copied region, where the copied region has been copied from the source storage to the destination storage, wherein the to-be-copied region is going to be copied from the source storage to the destination storage;

intercepting the virtual machine IO requests from the virtual machine, where the virtual machine IO requests from the virtual machine comprise write requests to the virtual machine disk image and read requests to the virtual machine disk image, wherein the write requests to the virtual machine disk image comprise write requests to the copied region and write requests to the to-be-copied region, wherein the read requests to the virtual machine disk image comprise read requests to the copied region and read requests to the to-be-copied region;

issuing write requests to the to-be-copied region and read requests to the to-be-copied region to the source storage;

recording the write requests to the copied region in a bitmap;

issuing the write requests to the copied region to the destination storage;

wherein for each read request to the copied region,

(i) checking the bitmap to determine whether a data block corresponding to the each read request to the copied region has been updated by one or more write requests to the virtual machine disk image;

(ii) if the data block corresponding to the each read request to the copied region has been updated by one or more write requests to the virtual machine disk image, reading the data block corresponding to the each read request to the copied region from the destination storage; and

(iii) if the data block corresponding to the each read request to the copied region has not been updated by one or more write requests to the virtual machine disk image, reading the data block corresponding to the each read request to the copied region from the source storage,

(c) copying the plurality of data blocks of the virtual machine disk image from the source storage to the destination storage;

partitioning the virtual machine disk image into a copied region and a to-be-copied region, where the copied region has been copied from the source storage to the destination storage, wherein the to-be-copied region is going to be copied from the source storage to the destination storage;

intercepting the virtual machine IO requests from the virtual machine, wherein the virtual machine IO requests from the virtual machine comprise write requests to the virtual machine disk image and read requests to the virtual machine disk image, wherein the write requests to the virtual machine disk image comprise write requests to the copied region and write requests to the to-be-copied region, wherein the read requests to the virtual machine disk image comprise read requests to the copied region and read requests to the to-be-copied region;

issuing write requests to the to-be-copied region and read requests to the to-be-copied region to the source storage;

issuing read requests to the copied region to the source storage;

wherein for each write request to the copied region,

(i) checking to determine a number of pending write requests to the copied region, wherein the number of pending write requests to the copied region is a number of write requests to the copied region that have been issued to the source storage and issued to the destination storage minus a number of the write requests to the copied region issued to source storage and issued to destination storage where both the write request to the copied region issued to the source storage and the write request to the copied region issued to the destination storage have been accomplished,

(ii) if the number of pending write requests to the copied region is less than or equal to a threshold number, issuing the each write request to the copied region to the source storage and issuing the each write request to the copied region to the destination storage;

(iii) if the number of pending write requests to the copied region is greater than the threshold number,

(a) delaying; and

(b) repeating (i), (ii), and (iii) until the each write request to the copied region is issued to the source storage and issued to the destination storage,

wherein when the each write request to the copied region issued to the source storage is accomplished or the each write request to the copied region issued to the destination storage is accomplished, marking the each write request to the copied region as completed and returning the each write request to the copied region marked as completed to the virtual machine.

2. The method according to claim 1, wherein the bitmap is on a memory, wherein the memory is external to the source storage, wherein the memory is external to the destination storage.

3. The method according to claim 1, wherein in (c), further comprising:

incrementing a counter for each write request to the copied region issued to the source storage and issued to the destination storage; and

decrementing the counter when both the each write request to the copied region issued to the source storage is accomplished and the each write request to the copied region issued to the destination storage is accomplished,

wherein checking to determine a number of pending write requests to the copied region comprises;

checking the counter, wherein the counter tracks the number of pending write requests to the copied region.

4. The method according to claim 3, wherein incrementing the counter occurs prior to issuing the each write request to the copied region to the source storage and issuing the each write request to the copied region to the destination storage.

5. The method according to claim 1, wherein in (b), accomplishing such intercepted write requests to the virtual machine disk image and such intercepted read requests to the virtual machine disk image after the data blocks corresponding to such intercepted write requests to the virtual machine disk image and such intercepted read requests to the virtual machine disk image have been copied from the source storage to the destination storage comprises:

deferring such intercepted write requests to the virtual machine disk image and such intercepted read requests to the virtual machine disk image;

putting such intercepted write requests to the virtual machine disk image and such intercepted read requests to the virtual machine disk image into a queue; and

handling such intercepted write requests to the virtual machine disk image and such intercepted read requests to the virtual machine disk image after the data blocks corresponding to such intercepted write requests to the virtual machine disk image and such intercepted read requests to the virtual machine disk image have been copied from the source storage to the destination storage.

6. The method according to claim 1, wherein copying the virtual machine disk image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine is accomplished without service interruption to a running workload on the virtual machine.

7. The method according to claim 1, wherein the source storage is a source solid state drive (SSD) or a source hard disk drive (HDD), wherein the destination storage is a destination SSD or a destination HDD.

8. The method according to claim 7, wherein the source storage is the source SSD and the destination storage is the destination SSD.

9. The method according to claim 7, wherein the source storage is the source SSD and the destination storage is the destination HDD.

10. The method according to claim 7, wherein the source storage is the source HDD and the destination storage is the destination SSD.

11. The method according to claim 7, wherein the source storage is the source HDD and the destination storage is the destination HDD.

12. The method according to claim 1, wherein copying the virtual machine disk image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage comprises copying the virtual machine disk image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine via a one of (a), (b), and (c) having a lowest total cost of migration of a total cost of migration of (a), a total cost of migration of (b), and a total cost of migration of (c), where a total cost of migration, MC, is based on:

an IO penalty, λ

_IO;

a migration time factor, λ

_{migration time};

a wear out factor, λ

_{wear out}; and

a weight, γ

, as follows;

13. The method according to claim 12, further comprising:

calculating at least two of the following;

(1) the total cost of migration for (a);

(2) the total cost of migration for (b); and

(3) the total cost of migration for (c).

14. The method according to claim 13, wherein prior to calculating the at least two of (1), (2), and (3), further comprising:

determining the IO performance of the source storage;

determining the IO performance of the destination storage;

if the source storage is a source SSD, determining the estimate of the amount of data written to the source storage per time unit;

if the destination storage is a destination SSD, determining the estimate of the amount of data written to the destination storage per time unit;

determining the estimate of the amount of data written to the source storage per time unit plus the estimate of the amount of data written to the destination storage per time unit;

determining the estimate of the period of time to complete copying the virtual machine disk image from the source storage to the destination storage and handling virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage; and

determining the amount of time copying the virtual machine disk image from the source storage to the destination storage would take if such copying were not interrupted.

15. The method according to claim 14, wherein migration time is determined from a size of the virtual machine disk image, the IO performance of the source storage, and the IO performance of the destination storage, and the estimate of the amount of data written to the source storage per time unit plus the estimate of the amount of data written to the destination storage per time unit, wherein the estimate of the amount of data written to the source storage per time unit plus the estimate of the amount of data written to the destination storage per time unit is determined from an estimate of workload data writes per time unit.

16. The method according to claim 12, further comprising:

receiving a size of the virtual machine disk image;

receiving an estimate of the workload data writes per time unit;

receiving an indication as to whether the source storage is a SSD or an HDD and the IO performance of the source storage, wherein the amount of data written to the source storage per time unit is the IO performance of the source storage;

receiving an indication as to whether the destination storage is a SSD or an HDD and the IO performance of the destination storage, wherein the amount of data written to the destination storage per time unit is the IO performance of the destination storage;

determining the estimate of the period of time to complete copying the virtual machine disk image from the source storage to the destination storage and handling virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage, wherein the estimate of the period of time to complete copying the virtual machine disk image from the source storage to the destination storage and handling virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage is determined from the size of the virtual machine disk image, the estimate of the workload data writes per time unit, the indication as to whether the source storage is a SSD or an HDD, the IO performance of the source storage, the indication as to whether the destination storage is a SSD or an HDD, and the IO performance of the destination storage; and

determining the amount of time copying the virtual machine disk image from the source storage to the destination storage would take if such copying were not interrupted from the size of the virtual machine disk image, wherein the amount of time copying the virtual machine disk image from the source storage to the destination storage would take if such copying were not interrupted from the size of the virtual machine disk image is determined from the size of the virtual machine disk image, the indication as to whether the source storage is a SSD or an HDD, the IO performance of the source storage, the indication as to whether the destination storage is a SSD or an HDD, and the IO performance of the destination storage.

17. The method according to claim 13, further comprising determining a lowest of the at least two of (1), (2), and (3) that were calculated, wherein the lowest total cost of migration is the lower of the at least two of (1), (2) and (3) that were calculated is the lowest total cost of migration.

18. The method according to claim 17, wherein the at least two of (1), (2), and (3) that were calculated is the total cost of migration for (a), the total cost of migration for (b), and the total cost of migration for (c).

19. The method according to claim 1, wherein copying the virtual machine disk image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage is accomplished via (a).

20. The method according to claim 19, wherein the destination storage is a destination SSD.

21. The method according to claim 1, wherein copying the virtual machine disk image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage is accomplished via (b).

22. The method according to claim 21, wherein the source storage is a SSD and the destination storage is a HDD.

23. The method according to claim 21, wherein issuing the write requests to the copied region to the destination storage keeps the copied region up-to-date without invoking a merge process.

24. The method according to claim 1, wherein copying the virtual machine disk image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage is accomplished via (c).

25. The method according to claim 24, wherein the source storage is a SSD and the destination storage is a HDD.

26. The method according to claim 24, wherein more than 50% of the write requests to the virtual machine are issued to the source storage during copying the virtual machine disk image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage.

27. The method according to claim 24, wherein at least a portion of the pending write requests to the copied region are completed when a virtual machine CPU is computating, when virtual machine memory data is being accessed, and/or when read requests to the virtual machine disk image are being accomplished.

28. The method according to claim 1, wherein in (c), further comprising:

when the source storage is a source SSD and the destination storage is a destination HDD,

incrementing a counter for each write request to the copied region issued to the destination storage; and

decrementing the counter when the each write request to the copied region issued to the destination storage is accomplished,

wherein checking to determine a number of pending write requests to the copied region comprises;

checking the counter, wherein the counter tracks the number of pending write requests to the copied region,

wherein in (c), when the source storage is a source HDD and the destination storage is a destination SSD,

incrementing a counter for each write request to the copied region issued to the source storage; and

decrementing the counter when both the each write request to the copied region issued to the source storage is accomplished,

wherein checking to determine a number of pending write requests to the copied region comprises;

checking the counter, wherein the counter tracks the number of pending write requests to the copied region.

29. The method according to claim 1, wherein with respect to (c), delaying comprises delaying for a certain period of time.

30. The method according to claim 1, wherein the source storage is a source disk array comprising a plurality of source array storages, wherein the destination storage is a destination disk array comprising a plurality of destination array storages, wherein each source array storage of the plurality of source array storages corresponds to a corresponding destination array storage of the plurality of destination array storages, such that the each source array storage is copied to the corresponding destination array storage, wherein each source array storage is copied to the corresponding destination array storage via a one of (a), (b), or (c) having the lowest total cost of migration with respect to the each source array storage and the corresponding destination array storage.

31. The method according to claim 12, wherein the lowest total cost of migration is lower than a total cost of migration for copying the virtual machine disk image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage via dirty block tracking, wherein the lowest total cost of migration is lower than a total cost of migration for copying the virtual machine disk image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage via IO mirroring.

32. The method according to claim 12, wherein migration time is the image copy time plus downtime, wherein the downtime is an amount of time the virtual machine is paused during copying the virtual machine disk image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage.

33. The method according to claim 19, wherein when the destination storage is a destination SSD storage, more than 50% of the write requests to the virtual machine disk image are issued to the destination storage during copying the virtual machine disk image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage.

34. The method according to claim 19, wherein when the destination storage is a destination SSD storage, at least 95% of the write requests are issued to the destination storage during copying the virtual machine disk image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage.

35. A method of migrating a virtual machine, comprising:

transferring a CPU state of a virtual machine and memory data of the virtual machine; and

transferring a virtual machine storage of the virtual machine, wherein transferring the virtual machine storage of the virtual machine is accomplished via the method of claim 1.

36. A virtual machine management system, comprising a non-volatile, computer readable media with instructions for performing the method of transferring a virtual machine storage of claim 1.

37. The method according to claim 1, wherein when the source storage is a source SSD and the destination storage is a destination HDD,

(i) if CO₂≦

γ

≦

1.0, copying the virtual machine image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage is accomplished via (a);

(ii) if CO₁≦

γ

<

CO₂, copying the virtual machine image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage is accomplished via (b); and

(iii) if O≦

γ

<

CO₁, copying the virtual machine image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage is accomplished via (c),

where CO₁ is a first cutoff and CO₂ is a second cutoff.

38. The method according to claim 37, wherein CO₁=0.3 and CO₂=0.6.

39. The method according to claim 1, wherein when the source storage is a source SSD and the destination storage is a destination HDD,

(i) if CO₁≦

γ

<

1.0, copying the virtual machine image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage is accomplished via (a); and

(ii) if O≦

γ

<

CO₁, copying the virtual machine image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage is accomplished via (b).

40. The method according to claim 39, wherein CO₁=0.6.

41. The method according to claim 1, wherein when the source storage is a source SSD and the destination storage is a destination HDD,

(i) if CO₁≦

γ

<

1.0, copying the virtual machine image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage is accomplished via (a); and

(ii) if O≦

γ

<

CO₁, copying the virtual machine image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage is accomplished via (c).

43. The method according to claim 1, wherein when the source storage is a source SSD and the destination storage is a destination HDD,

(i) if CO₁≦

γ

≦

1.0, copying the virtual machine image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage is accomplished via (b); and

(ii) if O≦

γ

<

CO₁, copying the virtual machine image from the source storage to the destination storage and handling the virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage is accomplished via (c).

44. The method according to claim 43, wherein CO₁=0.3.

45. The method according to claim 1, further comprising:

receiving a selection of (a), (b), or (c) from a user.

46. The method according to claim 12, further comprising receiving γ

from a user, wherein γ

is based on the user'"'"'s preference.

42. The method according to claim 42, wherein CO₁=0.5.

47. A method of transferring a virtual machine storage, comprising:

copying a virtual machine disk image of a virtual machine from a source storage to a destination storage, wherein the virtual machine disk image has a plurality of data blocks; and

handling virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage,

calculating at least two total costs of migration for a corresponding at least two ways to copy one or more data blocks of the virtual machine from the source storage to the destination storage and handle the virtual machine IO requests during copying the virtual disk image from the source storage to the destination storage, where a total cost of migration, MC, is based on;

an IO penalty, λ

_IO;

a migration time factor, λ

_{migration time};

a wear out factor, λ

_{wear out}; and

a weight, γ

, as follows;

48. A method of transferring a virtual machine storage, comprising:

copying a virtual machine disk image of a virtual machine from a source storage to a destination storage, wherein the virtual machine disk image has a plurality of data blocks; and

handling virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage,

calculating a total cost of migration to copy one or more data blocks of the virtual machine from the source storage to the destination storage and handle the virtual machine IO requests during copying the virtual disk image from the source storage to the destination storage, where a total cost of migration, MC, is based on;

an IO penalty, λ

_IO;

a migration time factor, λ

_{migration time};

a wear out factor, λ

_{wear out}; and

a weight, γ

, as follows;

49. The method according to claim 48, wherein the amount of data written to the source storage per time unit if the source storage is a source SSD is an estimate of the amount of data written to the source storage per time unit if the source storage is a source SSD, wherein the amount of data written to the destination storage per time unit if the destination storage is a destination SSD is an estimate of the amount of data written to the destination storage per time unit if the destination storage is a destination SSD, and wherein the period of time to complete copying the virtual machine disk image from the source storage to the destination storage and handling virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage is an estimate of the period of time to complete copying the virtual machine disk image from the source storage to the destination storage and handling virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage.

50. A method of determining a total cost of migration for transferring a virtual machine storage, comprising:

receiving information regarding a virtual machine disk image corresponding to a virtual machine, a source storage, and a destination storage; and

calculating a total cost of migration, where the total cost of migration, MC, is based on;

an IO penalty, λ

_IO;

a migration time factor, λ

_{migration time};

a wear out factor, λ

_{wear out}; and

a weight, γ

, as follows;

51. The method according to claim 50, wherein the amount of data written to the source storage per time unit if the source storage is a source SSD is an estimate of the amount of data written to the source storage per time unit if the source storage is a source SSD, wherein the amount of data written to the destination storage per time unit if the destination storage is a destination SSD is an estimate of the amount of data written to the destination storage per time unit if the destination storage is a destination SSD, and wherein the period of time to complete copying the virtual machine disk image from the source storage to the destination storage and handling virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage is an estimate of the period of time to complete copying the virtual machine disk image from the source storage to the destination storage and handling virtual machine IO requests from the virtual machine during copying the virtual machine disk image from the source storage to the destination storage.