On 06.05.2014 13:46, Hugo Mills wrote:
On Tue, May 06, 2014 at 01:26:44PM +0200, Hendrik Siedelmann wrote: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?You'd have to rewrite the chunk allocator to do this, _and_ provide different RAID levels for different subvolumes. The chunk/block group allocator right now uses only one rule for allocating data, and one for allocating metadata. Now, both of these are planned, and _might_ between them possibly cover the use-case you're talking about, but I'm not certain it's necessarily a sensible thing to do in this case.
But what does the allocator currently do when one disk runs out of space? I thought those disks do not get used but we can still write data. So the mechanism is already there, it just needs to be invoked when a drive is too busy instead of too full.
My question is, if you actually care about the performance of this
system, why are you buying some slow devices to drag the performance
of your fast devices down? It seems like a recipe for disaster...
Even the speed of a single hdd varies depending on where I write the data. So actually there is not much choice :-D. I'm aware that this could be a case of overengineering. Actually my first thought was to write a simple fuse module which only handles data and puts metadata on a regular filesystem. But then I thought that it would be nice to have this in btrfs - and not just for raid0.
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 writesIs this still true if I do very large writes? Or do those get broken down by the kernel somewhere?I guess it'll depend on the approach you use to do these "very large" writes, and on the exact definition of "very large". This is not an area I know a huge amount about. Hugo.
Never mind I'll just try it out! Hendrik -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
