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.
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.
Now I would imaging that one will be able to tune what's percentage of
ARC should keep compressed pages.
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.
--
Robert Milkowski
http://milek.blogspot.com
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