Hi all, I've been investigating a problem with tests of our database processing
jobs running on cephfs apparently reading several times more data than actually
exists in the working set. I haven't had much luck diagnosing the cause so far,
but one possibility I'd at least like to rule out is that the kernel cephfs
client is discarding pages with cached reads too aggressively and so we're
redundantly reading small chunks of data over and over again. Our access
patterns are quite unusual -- we read small chunks (perhaps a few hundred
kilobytes) from files, then close the file handles before reopening and doing
more reads. I found a 10 year old thread indicating that cephfs would outright
clear page caches for files in situations like this:
https://ceph-users.ceph.narkive.com/o2vJ4rWf/cephfs-and-page-cache
That behaviour was patched out long ago, but I'd like to know whether
aggressive page cache eviction like this is still known to be an issue with
certain access patterns, even if just to eliminate it as a possibility. We're
hosting ceph on the same machines as our database which further makes me wonder
if some kind of cache pressure could be involved.
Regards, Domhnall
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