On 11/07/13 14:25, Joel Sing wrote:
On Thu, 7 Nov 2013, Federico Giannici wrote:
For a decision I have to do, I have to know if the RAID1 implementation
in softraid evenly distributes the "read" load through all the disks.
Yes, reads are interleaved across all online chunks.
So, for example: with a two identical disks RAID1 implementation, can we
roughly assume that write speed is almost the same speed of a single
disk while the read speed is almost the double?
As you note below, it is not this simple... if each disk is on a separate
controller and there are no shared bottlenecks, this would be theoretically
close. Also, since you can have more than two chunks in a softraid RAID1
volume, you could theoretically increase the read speed by further
distributing it across disks/controllers.
Last I checked, I got nothing of the sort. It's been a while since,
indeed, but I believe the problem was that reading every other chunk
from a disk as opposed to reading each of them did not improve anything.
Still have to spin the disk etc.
I looked into making the selection of which chunk to read from a bit
smarter and succeeded in my limited setup (i.e. one process reading from
one physical disk and another processes reading from another), but only
as proof-of-concept.
This may differ on other types of disks, *especially* ssd disks, where I
would expect up to the double read capacity.
/Alexander
I know that reality is not so simple, but it's only to have an "ideal"
situation to understand the working of the system.
Right. Obviously benchmarking would be good starting point :)
