On Oct 2, 2009, at 5:05 AM, Robert Milkowski wrote:
Stuart Anderson wrote:
I am wondering if the following idea makes any sense as a way to
get ZFS to cache compressed data in DRAM?
In particular, given a 2-way zvol mirror of highly compressible
data on persistent storage devices, what would go wrong if I
dynamically added a ramdisk as a 3rd mirror device at boot time?
Would ZFS route most (or all) of the reads to the lower latency
DRAM device?
In the case of an un-clean shutdown where there was no opportunity
to actively remove the ramdisk from the pool before shutdown would
there be any problem at boot time when the ramdisk is still
registered but unavailable?
Note, this Gedanken experiment is for highly compressible (~9x)
metadata for a non-ZFS filesystem.
You would only get about 33% of IO's served from ram-disk.
With SVM you are allowed to specify a read policy on sub-mirrors for
just this reason, e.g.,
http://wikis.sun.com/display/BigAdmin/Using+a+SVM+submirror+on+a+ramdisk+to+increase+read+performance
Is there no equivalent in ZFS?
However at the KCA conference Bill and Jeff mentioned Just-in-time
decompression/decryption planned for ZFS. If I understand it
correctly some % of pages in ARC will be kept compressed/encrypted
and will be decompressed/decrypted only if accessed. This could be
especially useful to do so with prefetch.
I thought the optimization being discussed there was simply to avoid
decompressing/decrypting unused data. I missed the part about keeping
compressed data around in the ARC .
Now I would imaging that one will be able to tune what's percentage
of ARC should keep compressed pages.
That would be nice.
Now I don't remember if they mentioned L2ARC here but it would
probably be useful to have a tunable which would put compressed or
uncompressed data onto L2ARC depending on it's value. Which approach
is better would always depends on a given environment and on where
an actual bottleneck is.
I agree something like this would be preferable to the SVM ramdisk
solution.
Thanks.
--
Stuart Anderson ander...@ligo.caltech.edu
http://www.ligo.caltech.edu/~anderson
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