Re: [SQU] Assistance on cache sie etc..

Ounsted, Toby wrote:
>
> Sorry to bother you.. Can you explain (briefly) why striped RAID
> is so bad? Are you talking about Hw or Sw RAID?

Actually it is not striping but parity that is bad for the workload
presented by Squid.

Squid causes random writes of small blocks of data. If you read any
paper describing the characteristics of the different RAID levels you
will notice that this is the worst possible load pattern for a RAID5, as
every write operation effectively involves all disks in the stripe to
recalculate the parity.

RAID5 is optimal for large writes and random reads. Provides single disk
fault tolerance at the cost of one extra drive. Worst case pattern is
small random writes, where the performance can get degraded to that of
only slightly more than one single disk.

RAID0 (striping only) handles most kinds of work loads, but it's main
benefit is seen on large reads or writes. No fault tolerance. However,
Squid does a quite good job at distributing the workload on multiple
drives without the help of striping.

RAID1 (mirroring) doubles the hardware cost, but does not add any (or
very small) write penalty. Read performance is increased (up to
doubled).

--
Henrik Nordstrom
Squid Hacker
--
To unsubscribe, see http://www.squid-cache.org/mailing-lists.html