zfs discuss - Nov 2009 - mirroring ZIL device

Hi All,

Sorry if this question is already addressed in the documentation, but I am still
unclear about some details ZIL devices.

I am looking at provisioning some network attached storage with ZFS on the back
end.

In the interests of the ''inexpensive'' part of the acronym
''RAID'', I am looking at using SATA drives for the primary
storage striped over multiple mirrors.

I will be doing the operating system on a separate set of traditionally mirrored
15K SAS disks (formatted UFS2).

In the interests of performance given the SATA drives for the main storage, I am
considering provisioning a separate ZIL device, again on 15K SAS.

For the ZIL device, my understanding is that if it fails, everything still
''just works''.  Meanwhile, I tend to be a bit paranoid at times
and am curious about having the ZIL device be mirrored.

I suppose with hardware RAID, it would be obvious that I could just mirror two
15K SAS drives that way, export it out as a single device to the operating
system and I am clear.


My question(s) are:

#1.  Is it worthwhile to allocate a fast ZIL device to help out with writes to
the sluggish SATA main storage drives?  It seems to me that probably this kind
of case might be exactly what ZIL is for?

#2.  Is it possible for me to mirror those two 15K SAS drives I want to use for
ZIL directly within ZFS?  From the documentation, it seems not, and if I want to
go this way, I''ll want hardware RAID1.

#3.  Does anybody have any field/production information about just using low-end
(but newer generation) SATA SSDs for the ZIL device?  My concern here is
obviously about performance degradation over time with SSD, particularly as an
SSD device.  (Again, small budget, I can''t afford any of the fancy SSD
stuff).


Thanks,

- Mike

# 1. It may help to use 15k disks as the zil. When I tested using three 15k
disks striped as my zil, it made my workload go slower, even though it seems
like it should have been faster. My suggestion is to test it out, and see if it
helps.

#3. You may get good performance with an inexpensive SSD because the SSD should
have fast random writes, but probably not fast sequential writes. But I would
test it first against your anticipated workload. :) An Intel 32G X25-E runs just
shy of $400, and they are pretty speedy. I don''t know if that would fit
your budget. There is also some concern about losing power and having the X25
RAM cache disappear during a write.

-Scott
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