[ZFS] Using SSD with partitions

[Luchesar V. ILIEV]

On 16/10/2011 19:17, Daniel Kalchev wrote:
> 
> On Oct 16, 2011, at 17:16 , Luchesar V. ILIEV wrote:
> 
>> 6. If, OTOH, you're running a reasonably recent -STABLE (8 or 9),
>> then your zpool version is likely 28 (thanks, pjd@), which means
>> ZIL is not that scary, but you might still lose some data. Even an
>> unexpected power failure might cause trouble, unless the SSD is
>> designed to handle it gracefully (this typically involves some sort
>> of capacitor).
> 
> Just for the record: even without ZIL, you will most definitely lose
> data at power outage. In most cases, this will not damage the ZFS
> filesystem, but data will be lost. There is nothing that can prevent
> this.
> 
> Therefore, with ZFS v28, adding ZIL does not introduce any more risk
> to your data.

I might be wrong in my interpretation, but from what I remember, when
the power goes down, an unprotected SSD is likely to lose _more_ data
than simply its write buffers -- that's quite unlike a hard-drive. So
much, in fact, that the whole ZIL might become corrupted (and that's
potentially way more data than any device cache).

_If_ that's true, then isn't an array of only "conventional" HDDs, where
the ZIL is interleaved with the zpool itself, at least a bit safer from
power failures? Again, if we are taking the cheaper SSDs into account.

> One thing to have in mind is ZIL will help only under certain
> workloads, sequential write is not one of these. It helps most with
> database-type loads and sync writes like an NFS server that is
> written heavily. Freddie have good advice on determining if it will
> help.
> 
> L2ARC on the other hand may help enormously, especially if the spool
> is big. Workstation-class motherboards until recently were topped at
> 8GB RAM and ZFS is happy with as much RAM as you can offer. Adding
> L2ARC may provide more headroom. Benefits of course depend on the
> workload. Neither L2ARC or ZIL provide magical benefits.

Which is yet another reason to go for more RAM, as it tends to be quite
magic-yielding. Just kidding here, but, seriously, if Patrick has room
for some RAM upgrade, I think he should consider this, at least for
performance (a boot and OS drive, obviously, are another matter).

Cheers,
Luchesar