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There are three ways you can use SSDs with ZFS. First, you can use SSDs in place of rotating rust. Second, as was previously mentioned, you can use an SSD as

by dprice1 16y ago

There are three ways you can use SSDs with ZFS. First, you can use SSDs in place of rotating rust. Second, as was previously mentioned, you can use an SSD as an extension of your filesystem cache. That's called Level-2 Adaptive Replacement Cache (L2ARC). You can also use an SSD as a logging disk to accelerate writes; that's called "Logzilla", IIRC. In commercial offerings built around these features, you typically source an SSD that is either write-biased (logzilla) or read-biased, depending on what you are trying to do.

zdw 16y ago
Just some formal clarifications: Logzilla's formal name is ZIL, ZFS Intent Log. Generally these are SLC flash SSD's, mirrored as losing the ZIL on a ZFS pool can lead to "interesting" recovery situations. L2ARC is basically an extension of main memory used to cache data from the drives. If you lose L2ARC, there aren't any serious consequences. Usually L2ARC is implemented with less expensive MLC flash SSD's. On a related note, Seagate sells a Hybrid SSD+rust drive called the Momentus XT which uses it's 4GB of flash a similar manner to the L2ARC.
- ssmoot 16y ago
  I'm pretty sure the ZIL is not mirrored. If it were, it wouldn't increase performance since you'd still have to wait for the disks to fsync their copy to guarantee reliability. This is why Sun Storage products (and Nexenta, or homegrown clusters) put the ZIL SSDs in the JBODs and not the storage heads. You could of-course create a pool that did use a disk as the ZIL, but again, there'd be no point since the mirror has to be kept consistent otherwise it's worthless. Since the cache (L2ARC) is just cache, and you can lose it at anytime without data-loss, it is in the heads. Nit-picking. To be clear, the ZIL can (and should be) mirrored, you just want to do it with SSDs consistently. Mixing devices is possible, but you'll be limited to the performance of the slowest device. You can also create storage clusters with multiple heads. Again, a good idea IMO, but the ZIL is a critical component of the FS. If you lose it, you lose the whole pool (as a rule I think, but there might be clever hacks around that under special circumstances if you're lucky). So if you're going to multi-home your zpool, make sure the ZILs are in the shared-storage region. It's no good having redundant controllers if your ZIL is on the dead one.

by dprice1 16y ago

zdw 16y ago

Just some formal clarifications: Logzilla's formal name is ZIL, ZFS Intent Log. Generally these are SLC flash SSD's, mirrored as losing the ZIL on a ZFS pool can lead to "interesting" recovery situations. L2ARC is basically an extension of main memory used to cache data from the drives. If you lose L2ARC, there aren't any serious consequences. Usually L2ARC is implemented with less expensive MLC flash SSD's. On a related note, Seagate sells a Hybrid SSD+rust drive called the Momentus XT which uses it's 4GB of flash a similar manner to the L2ARC.

ssmoot 16y ago
I'm pretty sure the ZIL is not mirrored. If it were, it wouldn't increase performance since you'd still have to wait for the disks to fsync their copy to guarantee reliability. This is why Sun Storage products (and Nexenta, or homegrown clusters) put the ZIL SSDs in the JBODs and not the storage heads. You could of-course create a pool that did use a disk as the ZIL, but again, there'd be no point since the mirror has to be kept consistent otherwise it's worthless. Since the cache (L2ARC) is just cache, and you can lose it at anytime without data-loss, it is in the heads. Nit-picking. To be clear, the ZIL can (and should be) mirrored, you just want to do it with SSDs consistently. Mixing devices is possible, but you'll be limited to the performance of the slowest device. You can also create storage clusters with multiple heads. Again, a good idea IMO, but the ZIL is a critical component of the FS. If you lose it, you lose the whole pool (as a rule I think, but there might be clever hacks around that under special circumstances if you're lucky). So if you're going to multi-home your zpool, make sure the ZILs are in the shared-storage region. It's no good having redundant controllers if your ZIL is on the dead one.