Re: [squid-users] Re: Squid 1.2 formats and other Q's.

Chris Tilbury wrote:

> Many performance gurus recommend using a RAID0 stripe with suitable
> interleave to gain optimal disk read performance from a cluster of disks.
> However, this doesn't help the write situation that much.

Striping does not help random I/O much, compared to random I/O spread
on a number of separate disks. The benefits from striping is mostly
seen on sequential read/writes.

Squids I/O pattern is mostly random.

Squid will distribute the load on as many disks as is suitable, hence
striping will not be needed to get the I/O load evenly distributed on
many disks.

What's left of your design is filesystem logging, and as you said this
can be done for single disks as well as stripes. No need for striping
there either.

But until all this is finished, one large stripe with transaction log
is about the most efficient.

1 Stripe with log
2a Stripe without log
2b Multiple disks with log
3 Multiple disks without log

In case 2b (multiple disks with log) i'd recommend to have the log
mirrored to provide the most crash recovery with as few resources
as possible.
  * Log disk failure -> nothing lost
  * Data disk failure -> cache on that disk lost
Without mirroring of the log you may find youself in the situation
that none of you cache disks checks clean after a system failure where
the log disk crashed (caused by power failure or similar events).

---
Henrik Nordström
Sparetime Squid Hacker