You can also skip guides that suggest GB or GB drives. While you may not need that much space, performance on smaller capacity drives suffers. Another aspect to this is that you do not necessarily need power loss protection on an L2ARC drive as it is used as a read cache. You can also do without redundancy, especially if you can tolerate downtime to replace a failed device. We have some legacy recommendations. However, the fact remains that in case of system failure or power loss, all the buffered writes in the main memory are lost.
So applications which desire consistency over performance can open files in synchronous mode and then the data is only considered to be written once it is actually on the disk. Most databases, and applications like NFS, do rely on synchronous writes all the time. Of course, you may desire to have a good performance regardless of whether or not the files are in synchronous mode. By default some small amount of storage is always carved out from the pool to act like ZIL, even when you are using just a bunch of spinning disks for your storage.
The small and fast storage can be used as a Separate Intent Log or SLOG , which is where the newly arrived data would be stored temporarily before being flushed to the larger main storage of the pool.
To add a slog device run the command:. Search Everywhere Threads This forum This thread. Search titles only. Search Advanced search…. Everywhere Threads This forum This thread. Search Advanced…. New posts. Search forums. Install the app.
Thread starter achimcc Start date Jan 11, JavaScript is disabled. For a better experience, please enable JavaScript in your browser before proceeding. You are using an out of date browser.
It may not display this or other websites correctly. You should upgrade or use an alternative browser. Jan 11, 8 0 1 Synchronous writes are meant to ensure data consistency, but they come at the cost of performance. Synchronous writes: need to be acknowledged to have been written to persistent storage media before the write is seen as complete.
When the client sends out a write request for a synchronous write, it is still first sent to the RAM just like an asynchronous write, but the server will not acknowledge that the write has completed until it has been logged in the ZFS intent log ZIL.
Once the ZIL has been updated, then the write is committed and acknowledged. The ZIL exists as a portion of your storage pool by default which means the drive heads needs to physically move location to both update the ZIL and actually store the data as part of the pool, further impacting performance. Waiting for slower storage media HDDs causes some performance issues, especially from small random writes. This way if the data on the RAM was lost because of a power failure, the system would check the ZIL next time it was back on and find the data it was looking for.
Alternatively the data may be immediately written to disk along with pointers to it saved on the ZIL. The data would have its metadata updated after the next TXG was sent through to point to the correct location. If a power failure occurred, the server would check the ZIL and find out where the data was.
The system wasn't actually aware of where the data was housed until the next transaction group went through and would need to check the ZIL because the metadata wouldn't show where it was. One thing to note with an SLOG is it is generally best to mirror to allow it to serve its job of ensuring data consistency in the event of a power failure.
The impact performance of an SLOG will depend on the application. For small IO there will be a large improvement and could be a fair improvement on sequential IO as well. For a lot of synchronous writes such as use cases like database servers or hosting VMs it could also be helpful. However, the SLOGs primary function is not as a performance boon, but to save data that would otherwise be lost in the event of a power failure.
For mission critical applications, it could potentially be quite costly to lose the 5 seconds of data that would have been sent over in the next transaction group. That's also why an SLOG isn't truly a cache, it is a log like its name suggests.
0コメント