On 06.05.2014 13:19, Hugo Mills wrote:
On Tue, May 06, 2014 at 01:14:26PM +0200, Hendrik Siedelmann wrote:
On 06.05.2014 12:59, Hugo Mills wrote:
On Tue, May 06, 2014 at 12:41:38PM +0200, Hendrik Siedelmann wrote:
Hello all!
I would like to use btrfs (or anyting else actually) to maximize raid0
performance. Basically I have a relatively constant stream of data that
simply has to be written out to disk. So my question is, how is the block
allocator deciding on which device to write, can this decision be dynamic
and could it incorporate timing/troughput decisions? I'm willing to write
code, I just have no clue as to how this works right now. I read somewhere
that the decision is based on free space, is this still true?
For (current) RAID-0 allocation, the block group allocator will use
as many chunks as there are devices with free space (down to a minimum
of 2). Data is then striped across those chunks in 64 KiB stripes.
Thus, the first block group will be N GiB of usable space, striped
across N devices.
So do I understand this correctly that (assuming we have enough space) data
will be spread equally between the disks independend of write speeds? So one
slow device would slow down the whole raid?
Yes. Exactly the same as it would be with DM RAID-0 on the same
configuration. There's not a lot we can do about that at this point.
So striping is fixed but which disk takes part with a chunk is dynamic?
But for large workloads slower disks could 'skip a chunk' as chunk
allocation is dynamic, correct?
There's a second level of allocation (which I haven't looked at at
all), which is how the FS decides where to put data within the
allocated block groups. I think it will almost certainly be beneficial
in your case to use prealloc extents, which will turn your continuous
write into large contiguous sections of striping.
Why does prealloc change anything? For me latency does not matter, only
continuous troughput!
It makes the extent allocation algorithm much simpler, because it
can then allocate in larger chunks and do more linear writes
Is this still true if I do very large writes? Or do those get broken
down by the kernel somewhere?
I would recommend thoroughly benchmarking your application with the
FS first though, just to see how it's going to behave for you.
Hugo.
Of course - it's just that I do not yet have the hardware, but I plan to
test with a small model - I just try to find out how it actually works
first, so I know what look out for.
Good luck. :)
Hugo.
Thanks!
Hendrik
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