Neil Brown wrote:
On Wednesday May 30, jnelson-linux-raid@xxxxxxxxxxx wrote:
On Wed, 30 May 2007, Jon Nelson wrote:
On Thu, 31 May 2007, Richard Scobie wrote:
Jon Nelson wrote:
I am getting 70-80MB/s read rates as reported via dstat, and 60-80MB/s as
reported by dd. What I don't understand is why just one disk is being used
here, instead of two or more. I tried different versions of metadata, and
using a bitmap makes no difference. I created the array with (allowing for
variations of bitmap and metadata version):
This is normal for md RAID1. What you should find is that for
concurrent reads, each read will be serviced by a different disk,
until no. of reads = no. of drives.
Alright. To clarify, let's assume some process (like a single-threaded
webserver) using a raid1 to store content (who knows why, let's just say
it is), and also assume that the I/O load is 100% reads. Given that the
server does not fork (or create a thread) for each request, does that
mean that every single web request is essentially serviced from one
disk, always? What mechanism determines which disk actually services the
request?
It's probably bad form to reply to one's own posts, but I just found
static int read_balance(conf_t *conf, r1bio_t *r1_bio)
in raid1.c which, if I'm reading the rest of the source correctly,
basically says "pick the disk whose current head position is closest".
This *could* explain the behavior I was seeing. Is that not correct?
Yes, that is correct.
md/raid1 will send a completely sequential read request to just one
device. There is not much to be gained by doing anything else.
md/raid10 in 'far' or 'offset' mode lays the data out differently and
will issue read requests to all devices and often get better read
throughput at some cost in write throughput.
The whole "single process" thing may be a distraction rather than a
solution, as well. I wrote a small program using pthreads which shared
reads of a file between N threads in 1k blocks, such that each read was
preceded by a seek. It *seemed* that these were being combined in the
block layer before being passed on to the md logic, and treated as a
single read as nearly as I could tell.
I did NOT look at actually disk i/o (didn't care), but rather only at
the transfer rate from the file to memory, which did not change
significantly from 1..N threads active, where N was the number of
mirrors. And RAID-10 did as well with one thread as several.
--
bill davidsen <davidsen@xxxxxxx>
CTO TMR Associates, Inc
Doing interesting things with small computers since 1979
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