Tomas Ögren wrote:
On 13 November, 2006 - Sanjeev Bagewadi sent me these 7,1K bytes:
Tomas,
comments inline...
arc::print struct arc
{
anon = ARC_anon
mru = ARC_mru
mru_ghost = ARC_mru_ghost
mfu = ARC_mfu
mfu_ghost = ARC_mfu_ghost
size = 0x6f7a400
p = 0x5d9bd5a
c = 0x5f6375a
c_min = 0x4000000
c_max = 0x2e82a000
hits = 0x40e0a15
misses = 0x1cec4a4
deleted = 0x1b0ba0d
skipped = 0x24ea64e13
hash_elements = 0x179d
hash_elements_max = 0x60bb
hash_collisions = 0x8dca3a
hash_chains = 0x391
hash_chain_max = 0x8
no_grow = 0x1
}
So, about 100MB and a memory crunch..
Interesting ! So, it is not the ARC which is consuming too much memory....
It is some other piece (not sure if it belongs to ZFS) which is causing
the crunch...
Or the other possibility is that ARC ate up too much and caused a near
crunch situation
and the kmem hit back and caused ARC to free up it's buffers (hence the
no_grow flag enabled).
So, it (ARC) could be osscillating between large caching and then
purging the caches.
You might want to keep track of these values (ARC size and no_grow flag)
and see how they
change over a period of time. This would help us understand the pattern.
I would guess it grows after boot until it hits some max and then stays
there.. but I can check it out..
And if we know it ARC which is causing the crunch we could manually
change the values of
c_max to a comfortable value and that would limit the size of ARC.
But in the ZFS world, DNLC is part of the ARC, right?
My original question was how to get rid of "data cache", but keep
"metadata cache" (such as DNLC)...
However, I would suggest
that you try it out on a non-production machine first.
By, default the c_max is set to 75% of physmem and that is the hard
limit. "c" is the soft limit and
ARC would try and grow upto 'c". The value of "c" is adjusted when there
is a need to cache more
but, it will never exceed "c_max".
Regarding the huge number of reads, I am sure you have already tried
disabling the VDEV prefetch.
If not, it is worth a try.
That was part of my original question, how? :)
/Tomas
On recent bits, you can set 'zfs_vdev_cache_max' to 1 to disable the
vdev cache.
eric
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