[OpenAFS] Performance issue with "many" volumes in a single /vicep?

[Jeffrey Altman]

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On 3/17/2010 7:41 PM, Derrick Brashear wrote:
> On Wed, Mar 17, 2010 at 12:50 PM, Steve Simmons <scs@umich.edu> wrote:
>> We've been seeing issues for a while that seem to relate to the number=
 of volumes in a single vice partition. The numbers and data are inexact =
because there are so many damned possible parameters that affect performa=
nce, but it appears that somewhere between 10,000 and 14,000 volumes perf=
ormance falls off significantly. That 40% difference in volume count resu=
lts in 2x to 3x falloffs for performance in issues that affect the /vicep=
 as a whole - backupsys, nightly dumps, vos listvol, etc.
>>
>> My initial inclination is to say it's a linux issue with directory sea=
rches, but before pursuing this much further I'd be interested in hearing=
 from anyone who's running 14,000 or move volumes in a single vicep. No, =
I'm not counting .backup volumes in there, so 14,000 volumes means 28,000=
 entries in the directory.
>=20
> Another possibility: there's a hash table which is taking the bulk of
> that that you then search linearly.

In the 1.4 series, the volume hash table size is just 128 which
would produce (assuming even distributions) average hash chains of
160 to 220 volumes per bucket given the number of volumes you
describe.  This is quite long.

In the 1.5 series, the volume hash table size defaults to 256
which would be an average hash bucket chain length of 80 to 108.

I would say that for the number of volumes that you are using
that you would want the hash table to be no smaller than 4096
which would bring the average hash chain length below 7 per bucket.

In the 1.5 series the size of the hash table can be configured
at run-time with the -vhashsize value.  In 1.4 you can modify
the definition of VOLUME_HASH_TABLE_SIZE in src/vol/volume.c
and rebuild.

Jeffrey Altman



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