[linux-lvm] What is a good stripe size?
weissel at netcologne.de
Sun Jun 17 21:36:27 UTC 2001
idsfa at visi.com wrote 38 lines:
> On Sat, Jun 16, 2001 at 03:29:16PM +0200, Urs Thuermann wrote:
> > Using the PVs on sda2 and sdb2 I created a single VG and I want to
> > create striped LVs on it now. My questions is, how large should I
> > choose the stripe size to achive optimal performance. If I choose it
> > too large, I will probably loose the win of striping.
> You lose the advantage of striping if the stripe size is on the order
> of the file size. You want stripes which will be narrower than most
> of the files you will be using.
I wonder if you are looking at a single file or general
For a single file you may gain reading speed (writing is less
critical as it is buffered); however with a stripe size below
file size you will need to move the heads of both (or even
more) disks, increasing latency, effectively slowing down
reads unless you have fairly large files.
Often with more than one disk and randomly distributed files
(e.g. through the use of stripes) random files are on different
disks, so you can (best case) read as many files at the same
time -- in parallel -- as you have disks. This is only true
for sufficiently large stripes, though. Else a single file
blocks many/all disks, forcing longer/extra seeks for the
other files requested in parallel.
In conclusion (IMHO):
- small stripes increase the latency even for small reads,
hurting throughput (and slowing the reads even when looking
at a single file).
- sufficiently large stripes allow both parallel reads of
small files or accelerated reads (at the cost of
extra -- in this case insignificant -- seek time) of
single large files.
- Systems where the I/O is not the bottleneck, i.e. parallel
reads or accelerated reads of large files won't help much,
will not profit from stripes while still increasing the
risk of data loss through HD/controller failure.
- With lvm you could pmove the most accessed blocks so that
they are spread over all disks, probably you could even
split those disks in 2 parts: the fast 'begin' (outer
edge) of the platter and most of the (slower) inner parts.
This would have much the same effect of stripes, but would
need more attention (you need to run a program, probably
even inactivating the LV) and probably have finer granularity.
 Your seek time rises -- on the average -- the more heads
are in use. With one head you get 1/3 full-seek time for
a random head and file locations. With two heads you need
to move both heads, chances are that one of them has a
longer way than the other.
If you look at the first head, the seek time will be the
same if the first head is further away --- which is more
likely the further it is away, i.e. the higher the seek
time already would be for a single disk system. If it
is close (which happens and is part of the 1/3 _average_
seek time) then it is quite likely that the other head is
further away -- thus the average seek time increases.
This is quite uncritical for large files, but with short
files the seek time is greater than the read time. But only
where the increased seek time is small against the read
time a reduced read time can do reduce overall time --
that is, only large files will be faster.
 small: read time << seek time, fits in one stripe
large: read time >> seek time, does certainly not fit in
 read time >> seek time
 all disks are 'single point of failure'. Most file systems
do not like loosing spots all over the place. But then you
do backup religiously, test your backups and have recovery
plans in place, yes?
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