SYSTEMS AND METHODS FOR A READ ONLY MODE FOR A PORTION OF A STORAGE SYSTEM

• US 20090248756A1
• Filed: 03/27/2008
• Published: 10/01/2009
• Est. Priority Date: 03/27/2008
• Status: Active Grant

• Current Assignee: Emc IP Holding Company LLC (Dell Technologies Inc.)
• Original Assignee: EMC Corporation (Dell Technologies Inc.)
• Inventors: Passey, Aaron J., Fachan, Neal T., Akidau, Tyler A.
• Granted Patent: US 7,949,636 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G06F 11/004 Error avoidance G06F11/07 and subgroups take precedence
• CPC Class Codes: G06F 11/0727 in a storage system, e.g. in a DASD or network based storage system drivers for digital recording or reproducing units G06F3/06; circuits for error detection or correction within digital recording or reproducing units G11B20/18; for distributed storage of data in networks, e.g. transport arrangements for network file system [NFS], storage area networks [SAN] or network attached storage [NAS], H04L67/1097
• CPC Class Codes: G06F 11/0793 Remedial or corrective actions recovery from an exception in an instruction pipeline G06F9/3861; by retry G06F11/1402; for recovering from a failure of a protocol instance or entity H04L69/40
• CPC Class Codes: G06F 11/1471 involving logging of persistent data for recovery
• CPC Class Codes: G06F 12/1416 by checking the object accessibility, e.g. type of access defined by the memory independently of subject rights G06F12/1458 takes precedence

Abstract

In general, embodiments of the invention relate to reading data from and writing data to a storage system. Specifically, embodiments of the invention relate to a read only mode for a portion of a storage system. In one embodiment, a selective read-only mode for a portion of a storage system is implemented by monitoring a condition that may affect a subset of persistent storage in a storage system, by detecting the condition, by entering a read-only mode for the subset, and by enforcing a policy of processing write requests and read requests to the storage system, which includes processing the write requests without modifying user data stored on the subset and processing the read requests, including requests for user data stored on the subset.

Claims

1. A method of implementing a selective read-only mode in a storage system with a transaction journal, comprising:

monitoring a condition that affects data stored in a transaction journal corresponding to a portion of a storage system, wherein the transaction journal records in a persistent storage transactions that modify user data stored in the portion;

detecting the condition;

entering a read-only mode for the portion of the storage system;

enforcing a policy of processing write requests and read requests to the storage system, comprising;

processing the write requests without modifying user data on the portion; and

processing the read requests, including requests for user data stored on the portion.

2. The method of claim 1, wherein the storage system is a distributed file system comprising a cluster of nodes, wherein the portion and the transaction journal correspond to one of the nodes.

3. The method of claim 1, wherein the transaction journal is stored in at least one of the following:

non-volatile random access memory (NVRAM), flash memory, hard-disk drive, compact-disc read-only memory (CD-ROM), digital versatile disc (DVD), optical storage, and magnetic tape drive.

4. The method of claim 1, wherein the condition is at least one of the following:

batteries supporting the persistent storage have been physically removed;

batteries supporting the persistent storage have started to fail;

a chassis housing the persistent storage has been opened;

a user has requested read-only mode via a software interface;

a user has requested read-only mode via a hardware interface;

a hardware failure that negatively affects the durability of the subset of persistent storage;

a condition that negatively affects the durability of the subset of persistent storage;

a condition that negatively affects a main power supply;

a condition that negatively affects backup flash memory; and

a condition related to auto-correcting single-bit errors for non-volatile memory components.

5. The method of claim 1, wherein entering a read-only mode for the portion occurs before the portion has been mounted into the storage system.

6. The method of claim 1, wherein entering a read-only mode for the portion occurs after the portion has been mounted into the storage system.

7. The method of claim 1, wherein entering the read-only mode comprises waiting until transactions in the transaction journal have resolved to at least one of an aborted and a committed state, marking the transaction journal as unwritable, and backing up the transaction journal.

8. The method of claim 1, wherein entering the read-only mode comprises marking the transaction journal as unwritable and backing up the transaction journal.

9. The method of claim 8, further comprising continuing to process unresolved transactions affecting the portion by marking in memory as commit deferred, without writing to the transaction journal, those transactions for which a commit message is received after entering the read-only mode.

10. The method of claim 9, further comprising processing those transactions marked commit deferred, after leaving the read-only mode for the portion, by marking each of those transactions as committed in the transaction journal, and sending committed messages to other affected portions of the storage system.

11. The method of claim 8, further comprising continuing to process unresolved transactions affecting the portion by storing in memory, without writing to the transaction journal, a queue of deferred writes to the transaction journal associated with an abort message for an unresolved transaction received after entering the read-only mode.

12. The method of claim 11, further comprising processing the deferred writes in the queue, after leaving the read-only mode for the portion, by writing the deferred writes in the transaction journal.

13. The method of claim 1, wherein processing the write requests without modifying user data stored on the affected subset comprises writing data, which is addressed to a location within the affected subset, to another location in the storage system and updating that data'"'"'s metadata to identify the new location.

14. A computer-readable medium having instructions stored thereon, the instructions comprising the method of claim 1.

32. The method of claim 31, further comprising continuing to process unresolved transactions affecting the portion by storing in memory, without writing to the transaction journal, a queue of deferred writes to the transaction journal associated with an abort message for an unresolved transaction received after entering the read-only mode for the portion.

33. The method of claim 32, further comprising processing the deferred writes in the queue, after leaving the read-only mode for the portion, by writing the deferred writes in the transaction journal.

15. A distributed storage system configured to implement a selective read-only mode, comprising:

a plurality of storage modules configured to communicate via a network, each of the plurality of storage modules configured to process at least one of read and write requests on behalf of the entire distributed storage system, wherein the storage modules configured to handle write requests are configured to individually enter a read-only mode if at least one condition is detected that affects data stored in a transaction journal corresponding to a portion of a storage system, wherein the transaction journal records in persistent storage transactions that modify user data stored in the portion; and

an allocation module configured to exclude storage modules operating in read-only mode from processing write requests.

16. The distributed storage system of claim 15, wherein the storage system is a distributed file system, and wherein file data and metadata of the file system are distributed across the plurality of storage modules.

17. The distributed storage system of claim 15, wherein the transaction journal is stored in at least one of the following:

non-volatile random access memory (NVRAM), flash memory, hard-disk drive, compact-disc read-only memory (CD-ROM), digital versatile disc (DVD), optical storage, and magnetic tape drive.

18. The distributed storage system of claim 15, wherein the at least one condition is at least one of the following:

batteries supporting the persistent storage have been physically removed;

batteries supporting the persistent storage have started to fail;

a chassis housing the persistent storage has been opened;

a user has requested read-only mode via a software interface;

a user has requested read-only mode via a hardware interface;

a hardware failure that negatively affects the durability of the subset of persistent storage;

a condition that negatively affects the durability of the subset of persistent storage;

a condition that negatively affects a main power supply;

a condition that negatively affects backup flash memory; and

a condition related to auto-correcting single-bit errors for non-volatile memory components.

19. The distributed storage system of claim 15, the allocation module further configured to write data, which is addressed to a location within the portion, to another location in the storage system and to update that data'"'"'s corresponding metadata to identify the new location.

20. A storage module configured to communicate with other storage modules and to enter a read-only mode, comprising:

a first persistent storage with user data stored thereon;

a second persistent storage with transaction data stored thereon;

memory with temporary data stored thereon;

a monitoring module configured to detect at least one condition that affects the second persistent storage;

a read-only module configured to place the storage module into a read-only mode after the monitoring module detects the at least one condition.

21. The storage module of claim 20, wherein the first persistent storage and the second persistent storage are, respectively, at least one of the following:

non-volatile random access memory (NVRAM), flash memory, hard-disk drive, compact-disc read-only memory (CD-ROM), digital versatile disc (DVD), optical storage, and magnetic tape drive.

22. The storage module of claim 20, wherein the at least one condition is at least one of the following:

batteries supporting the second persistent storage have been physically removed;

batteries supporting the second persistent storage have started to fail;

a chassis housing the second persistent storage has been opened;

a user has requested read-only mode via a software interface;

a user has requested read-only mode via a hardware interface;

a hardware failure that negatively affects the durability of the subset of persistent storage;

a condition that negatively affects the durability of the subset of persistent storage;

a condition that negatively affects a main power supply;

a condition that negatively affects backup flash memory; and

a condition related to auto-correcting single-bit errors for non-volatile memory components.

23. The storage module of claim 20, wherein the read-only module is configured to place the storage module into a read-only mode before the storage module has been mounted into the distributed storage system.

24. The storage module of claim 20, wherein the read-only module is configured to place the storage module into a read-only mode after the storage module has been mounted into the distributed storage system.

25. The storage module of claim 20, wherein the read-only module is further configured to place the storage module into a read-only mode by waiting until transactions affecting the persistent storage have sufficiently resolved, marking the persistent storage as unwritable, and backing up the persistent storage.

26. The storage module of claim 20, wherein the read-only module is further configured to place the storage module into a read-only mode by marking the second persistent storage as unwritable and backing up the transaction data.

27. The storage module of claim 26, wherein the read-only module is further configured to process unresolved transactions affecting the first persistent storage by marking in the memory as commit deferred, without writing to the second persistent storage, those transactions for which a commit message is received after entering the read-only mode.

28. The storage module of claim 27, wherein the read-only module is further configured to process those transactions marked commit deferred, after leaving the read-only mode, by marking each of those transactions as committed in the transaction journal, and sending committed messages to other affected storage modules.

29. The storage module of claim 26, wherein the read-only mode is further configured to process unresolved transactions affecting the first persistent storage by storing in memory, without writing to the second persistent storage, a queue of deferred writes to the second persistent storage associated with an abort message for an unresolved transaction received after entering the read-only mode.

30. The storage module of claim 29, wherein the read-only mode is further configured to process the deferred writes in the queue, after leaving the read-only mode, by writing the deferred writes in the second persistent storage.

31. A method of entering a read-only mode for a portion of a storage system, comprising:

marking as unwritable a transaction journal associated with a portion of a storage system entering a read-only mode;

backing up the transaction journal without writing to the portion;

continuing to process unresolved transactions affecting the portion by marking as commit deferred, without writing to the transaction journal, those transactions for which a commit message is received after entering the read-only mode; and

processing those transactions marked commit deferred, after leaving the read-only mode for the portion, by marking each of those transactions as committed in the transaction journal, and sending committed messages to other affected portions of the storage system.

34. A computer-readable medium having instructions stored thereon, the instructions comprising the method of claim 31.