On Sat, 21 Jan 2012, Jim Klimov wrote:


Regarding the written data, I believe it may find place in the
ARC, and a for the past few TXGs it could still remain there.

Any data in the ARC is subject to being overwritten with updated data just a millisecond later. It is a live cache.

I am not sure it is feasible to "guarantee" that it remains in
RAM for a certain time. Also there should be a way to enforce
media reads and not ARC re-reads when verifying writes...

Zfs already knows how to by-pass the ARC. However, any "media" reads are subject to caching since the underlying devices try very hard to cache data in order to improve read performance.

As an extreme case of caching, consider a device represented by an iSCSI LUN on a OpenSolaris server with 512GB of RAM. If you request to read data you are exceedingly likely to read data from the zfs ARC on that server rather than underlying "media".

Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
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