Gary Mills wrote:
I've been watching the ZFS ARC cache on our IMAP server while the
backups are running, and also when user activity is high. The two
seem to conflict. Fast response for users seems to depend on their
data being in the cache when it's needed. Most of the disk I/O seems
to be writes in this situation. However, the backup needs to stat
all files and read many of them. I'm assuming that all of this
information is also added to the ARC cache, even though it may never
be needed again. It must also evict user data from the cache, causing
it to be reloaded every time it's needed.
We use Networker for backups now. Is there some way to configure ZFS
so that backups don't churn the cache? Is there a different way to
perform backups to avoid this problem? We do keep two weeks of daily
ZFS snapshots to use for restores of recently-lost data. We still
need something for longer-term backups.
Hi Gary,
Find out whether you have a problem first. If not, don't worry, but
read one. If you do have a problem, add memory or an L2ARC device.
The ARC was designed to mitigate the effect of any single burst of
sequential I/O, but the size of the cache dedicated to more Frequently
used pages (the current working set) will still be reduced, depending
on the amount of activity on either side of the cache.
As the ARC maintains a shadow list of recently evicted pages from both
sides of the cache, such pages that are accessed again will then return
to the 'Frequent' side of the cache.
There will be continuous competition between 'Recent' and 'Frequent'
sides of the ARC (and for convenience, I'm glossing over the existence
of 'Locked' pages).
Several reasons might cause pathological behaviour - a backup process
might access the same metadata multiple times, causing that data to
be promoted to 'Frequent', flushing out application related data.
(ZFS does not differentiate between data and metadata for resource
allocation, they all use the same I/O mechanism and cache.)
On the other hand, you might just not have sufficient memory to keep
most of your metadata in the cache, or the backup process is just too
aggressive. Adding memory or an L2cache might help.
Cheers,
Henk
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