Systems and methods for a read only mode for a portion of a storage system
View Patent ↗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 affects 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.
1. 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 comprising at least one processor and 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 persistent storage on a respective storage module; and
an allocation module configured to exclude storage modules operating in read-only mode from processing write requests.
2. The distributed storage system of claim 1 , 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.
3. The distributed storage system of claim 1 , wherein the persistent storage is 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 distributed storage system of claim 1 , 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 persistent storage; a condition that negatively affects the durability of the 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 distributed storage system of claim 1 , the allocation module further configured to write data, which is addressed to a location within the persistent storage affected by the at least one condition, to another location in the storage system and to update that data's corresponding metadata to identify the new location.
6. The distributed storage system of claim 1 , wherein each of the storage modules configured to handle write requests is further configured to, while in read-only mode:
process the write requests without modifying data stored on the storage module; and
process the read requests, including read requests for data stored on the storage module.
7. The distributed storage system of claim 1 , wherein the allocation module is further configured to:
receive a request to write data to a data block on one of the storage modules;
determine whether the data block is on a storage module which is in the read-only mode; and
if the data block is on a storage module which is in the read-only mode, write the data to one or more of the plurality of storage modules which is not in the read-only mode.
8. The distributed storage system of claim 7 , wherein the allocation module is further configured to update metadata associated with the data.
9. The distributed storage system of claim 1 , further comprising a monitor module configured to detect the at least one condition that affects persistent storage.
10. The distributed storage system of claim 9 , wherein the monitor module is further configured to maintain an indication of all existing conditions that affect persistent storage which the monitor module has detected.
11. The distributed storage system of claim 1 , wherein the write requests are associated with transactions, each transaction affecting one or more of the plurality of storage modules.
12. The distributed storage system of claim 11 , wherein each of the storage modules configured to handle write requests is further configured to wait for one or more of the transactions to resolve before entering the read-only mode.
13. The distributed storage system of claim 11 , wherein each of the storage modules configured to handle write requests is further configured to maintain a data journal which maintains information about one or more of the transactions.
14. The distributed storage system of claim 13 , wherein each of the storage modules configured to handle write requests is further configured to designate the data journal unwritable before entering the read-only mode.
15. The distributed storage system of claim 13 , wherein each of the storage modules configured to handle write requests is further configured to create a copy of the data journal before entering the read-only mode.
16. The distributed storage system of claim 13 , wherein each of the storage modules configured to handle write requests is further configured to, while in the read-only mode, defer modifications to the data journal until after leaving the read-only mode.
17. The distributed storage system of claim 11 , wherein each of the storage modules configured to handle write requests is further configured to, while in the read-only mode, defer operations associated with resolving one or more of the transactions until after leaving the read-only mode.
18. The distributed storage system of claim 1 , wherein each of the storage modules configured to handle write requests is further configured to maintain a queue while in the read-only mode, the queue configured to comprise deferred write commands associated with transactions.
19. The distributed storage system of claim 18 , wherein each of the storage modules configured to handle write requests is further configured to maintain a list of transaction states for the transactions.
20. The distributed storage system of claim 18 , wherein each of the storage modules configured to handle write requests is further configured to execute one or more of the deferred write commands after leaving the read-only mode.