Re: RAIDframe performance vs. stripe size

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

> Edgar is describing the desideratum for a minimum-latency application.
Yes, I'm looking for minimum latency.
I've logged the current file server's disc "business" and the only time
they really are busy is during the nightly backup. I suppose that mainly
consists of random reads traversing the directory tree followed by random
reads transferring the modified files (which, I suppose, are typically small).

Of course I finally will have to experiment, but a) I'm used to trying to
understand the theory first and b) the rate of experiments will probably be
limited to one per day or less given the amount of real data I would have
to re-transfer each time.

Since my understanding of the NetBSD I/O system may be incorrect, incomplete
or whatever, I better try to ask people first who know better:

Given 6.0/amd64 and a RAID 5 accross 4+1 SAS discs.
Suppose I have a 16k fsbsize and a stripe size such that there is one full
fs block per disc (i.e. the stripe is 4*16k large). Suppose everything
is correctly aligned (apart from where I say it isn't, of course).

Suppose there is no disc I/O apart from that I describe.

Scenario A:
I have two processes each reading a single file system block. Those two
blocks happen to end up on two different discs (there is a 3/4 probability
for that being true). Will I end up with those two discs actually seeking
in parallel?

Scenario B:
I have one process reading a 64k chunk that is 16k-aligned, but not 64k-
aligned (so it's not just reading a full stripe). Will I end up in four
discs seeking and reading in parallel? I.e. will this be degraded wrt. a
stripe-aligned read?

Scenario C:
I have one process doing something largely resulting in meta-data reads
(i.e. traversing a very large directory tree). Will the kernel only issue
sequential reads or will it be able to parallelise, e.g. reading indirect
blocks?

What will change if I scale all sizes by a factor of, say, 4, such that
the full stripe exceeds MAXPHYS?

Sorry if the answers to those question are obvious from what has already
been written so far for someone more familiar with the matter than I am.