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== Summary == Prior to this change, sync writes to a zvol are processed serially. This commit makes zvols process concurrently outstanding sync writes in parallel, similar to how reads and async writes are already handled. The result is that the throughput of sync writes is tripled. == Background == When a write comes in for a zvol (e.g. over iscsi), it is processed by calling `zvol_request()` to initiate the operation. ZFS is expected to later call `BIO_END_IO()` when the operation completes (possibly from a different thread). There are a limited number of threads that are available to call `zvol_request()` - one one per iscsi client (unless using MC/S). Therefore, to ensure good performance, the latency of `zvol_request()` is important, so that many i/o operations to the zvol can be processed concurrently. In other words, if the client has multiple outstanding requests to the zvol, the zvol should have multiple outstanding requests to the storage hardware (i.e. issue multiple concurrent `zio_t`'s). For reads, and async writes (i.e. writes which can be acknowledged before the data reaches stable storage), `zvol_request()` achieves low latency by dispatching the bulk of the work (including waiting for i/o to disk) to a taskq. The taskq callback (`zvol_read()` or `zvol_write()`) blocks while waiting for the i/o to disk to complete. The `zvol_taskq` has 32 threads (by default), so we can have up to 32 concurrent i/os to disk in service of requests to zvols. However, for sync writes (i.e. writes which must be persisted to stable storage before they can be acknowledged, by calling `zil_commit()`), `zvol_request()` does not use `zvol_taskq`. Instead it blocks while waiting for the ZIL write to disk to complete. This has the effect of serializing sync writes to each zvol. In other words, each zvol will only process one sync write at a time, waiting for it to be written to the ZIL before accepting the next request. The same issue applies to FLUSH operations, for which `zvol_request()` calls `zil_commit()` directly. == Description of change == This commit changes `zvol_request()` to use `taskq_dispatch_ent(zvol_taskq)` for sync writes, and FLUSh operations. Therefore we can have up to 32 threads (the taskq threads) simultaneously calling `zil_commit()`, for a theoretical performance improvement of up to 32x. To avoid the locking issue described in the comment (which this commit removes), we acquire the rangelock from the taskq callback (e.g. `zvol_write()`) rather than from `zvol_request()`. This applies to all writes (sync and async), reads, and discard operations. This means that multiple simultaneously-outstanding i/o's which access the same block can complete in any order. This was previously thought to be incorrect, but a review of the block device interface requirements revealed that this is fine - the order is inherently not defined. The shorter hold time of the rangelock should also have a slight performance improvement. For an additional slight performance improvement, we use `taskq_dispatch_ent()` instead of `taskq_dispatch()`, which avoids a `kmem_alloc()` and eliminates a failure mode. This applies to all writes (sync and async), reads, and discard operations. == Performance results == We used a zvol as an iscsi target (server) for a Windows initiator (client), with a single connection (the default - i.e. not MC/S). We used `diskspd` to generate a workload with 4 threads, doing 1MB writes to random offsets in the zvol. Without this change we get 231MB/s, and with the change we get 728MB/s, which is 3.15x the original performance. We ran a real-world workload, restoring a MSSQL database, and saw throughput 2.5x the original. We saw more modest performance wins (typically 1.5x-2x) when using MC/S with 4 connections, and with different number of client threads (1, 8, 32). Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com> Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Signed-off-by: Matthew Ahrens <mahrens@delphix.com> Closes #10163 |
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