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Re: [linux-lvm] RAID chunk size & LVM 'offset' affecting RAID stripe alignment



On Jun 21, 2010, at 12:26 AM, Linda A. Walsh wrote:
Revisiting an older topic (I got sidetracked w/other issues,
as usual, fortunately email usually waits...).

About a month ago, I'd mentioned docs for 2 HW raid cards
(LSI & Rocket Raid) both suggested 64K as a RAID chunk size.

Two responses came up, Doug Ledford said:
Hardware raid and software raid are two entirely different things
when it comes to optimization.

And Luca Berra said:
I think 64k might be small as a chunk size, depending on your
array size you probably want a bigger size.
(I asked why and Luca contiued..)

First we have to consider usage scenarios, i.e. average read and
average write size, large reads benefit from larger chunks,

Correction: all reads benefit from larger chunks now a days. The only reason to use smaller chunks in the past was to try and get all of your drives streaming data to you simultaneously, which effectively made the total aggregate throughput of those reads equal to the throughput of one data disk times the number of data disks in the array. With modern drives able to put out 100MB/s sustained by themselves, we don't really need to do this any more, and if we aren't attempting to get this particular optimization (which really only existed when you were doing single threaded sequential I/O anyway, which happens to be rare on real servers), then larger chunk sizes benefit reads because they help to ensure that reads will, as much as possible, only hit one disk. If you can manage to make every read you service hit one disk only, you maximize the random I/O ops per second that your array can handle.

small
writes with too large chunks would still result on whole stripe
Read-Modify-Write.

There is a very limited set of applications where the benefit of streaming writes versus a read-modify-write cycle are worth the trade off that they require. Specifically, only if you are going to be doing more writing to your array than reading, or maybe if you are doing at least 33% of all commands as writes, then you should worry about this. By far and away the vast majority of usage scenarios involve far more reads than writes, and in those cases you always optimize for reads. However, even if you are optimizing for writes, then what I wrote above about trying to make it so that your writes always only fall on one disk (excepting the fact that parity also needs updated) still holds true unless you can make your writes *reliably* take up the entire stripe. The absolute worst thing you could do is use a small chunk size thinking that it will cause your writes to skip the read-modify-write cycle and instead do a complete stripe write, then have your writes reliably only do half stripe writes instead of full stripe writes. A half stripe write is worse than a full stripe write, and is worse than a single chunk write. It is the worst case scenario.

there were people on linux-raid ml doing benchmarks, and iirc
using chunks between 256k and 1m gave better average results...

(Doug seconded this, as he was the benchmarker..)
 That was me.  The best results are with 256 or 512k chunk sizes.
Above 512k you don't get any more benefit.

------

My questions at this point -- why are SW and HW raid so different?
Aren't they doing the same algorithms on the same media?

Yes and no. Hardware RAID implementations provide a pseudo device to the operating system, and implement their own caching subsystem and command elevator algorithm on the card for both the pseudo device and the underlying physical drives. Linux likewise has its own elevator and caching subsystems that work on the logical drive. So, in the case of software raid you are usually talking the stack looks something like this:

filesystem -> cacheing layer -> block device layer with elevator for logical device -> raid layer -> block device layer with noop elevator for physical device -> scsi device layer -> physical drive

In the case of hardware raid controller, it's like this:

filesystem -> cacheing layer -> block device layer with elevator for logical device -> scsi layer -> raid controller driver -> hardware raid controller cacheing layer and elevator -> hardware raid controller raid stack -> hardware raid controller physical drive driver layer -> physical drive

So, while at a glance it might seem that they are implementing the same algorithms on the same devices, the details of how they do so are drastically different and hence the differences in optimal numbers.

FWIW, we don't generally have access to the raid stack on those hardware raid controllers to answer the question of why they perform best with certain block sizes, but my guess is that they have built in assumptions in the cacheing layer related to those block sizes, that result in them being hamstrung at other block sizes.

 SW might
be a bit slower at some things (or it might be faster if it's good
SW and the HW doesn't clearly make it faster).

Secondly, how would array size affect the choice for chunk size?

Array size doesn't affect optimal chunk size.

 Wouldn't chunk size be based on your average update size, trading
off against the increased benefit of a larger chunk size benefitting reads more than writes. I.e. if you read 10 times as much as write,
then maybe faster reads provide a clear win, but if you update
nearly as much as read, then a stripe size closer to your average
update size would be preferable.

See my comments above, but in general, you can always play it safe with writes and use a large chunk size so that writes generally are single chunk writes. If you do that, you get reasonably good writes, and optimal reads. Unless you have very strict control of the writes on your device, it's almost impossible to have optimal full-stripe writes, and if you try to aim for that, you have a large chance of failure. So, my advice is to not even try to go down that path.

Concerning the benefit of a larger chunk size benefitting reads --
would that benefit be less if one also was using read-ahead on the
array?

The benefit of large chunk size for reads is that it keeps the read on a single device as frequently as possible. Because readahead doesn't kick in immediately, it doesn't negate that benefit on random I/O, and on truly sequential I/O it turns out to still help things as it will start the process of reading from the next disk ahead of time but usually only after we've determined we truly are going to need to do exactly that.

>-----------------------<
In another note, Luca Berra commented, in response to my observation that my 256K-data wide stripes (4x64K chunks) would be skewed by a
chunk size on my PV's that defaulted to starting data at offset 192K:

LB> it will cause multiple R-M-W cycles fro writes that cross stripe
LB> boundary, not good.

I don't see how it would make a measurable difference.

Alignment of the lvm device on top of the raid device most certainly will make a measurable difference.

If it did, wouldn't we also have to account for the parity disks so that they are aligned as well -- as they also have to be written during a stripe-write? I.e. -- if it is a requirement that they be aligned,
it seems that the LVM alignment has to be:

(total disks)x(chunk-size)

not
(data-disks)x(chunk-size)

No. If you're putting lvm on top of a raid array, and the raid array is a pv to the lvm device, then the lvm device will only see (data- disks)x(chunk-size) of space in each stripe. The parity block is internal to the raid and never exposed to the lvm layer.

as I *think* we were both thinking when we earlier discussed this.

Either way, I don't know how much of an effect there would be if,
when updating a stripe, some of the disks read/write chunk "N", while
the other disks use chunk "N-1"...  They would all be writing 1
chunk/stripe update, no?

No. This goes to the heart of a full stripe write versus partial stripe write. If your pv is properly aligned on the raid array, then a single stripe write of the lvm subsystem will be exactly and optimally aligned to write to a single stripe of the raid array. So, let's say you have a 5 disk raid5 array, so 4 data disks and 1 parity disk. And let's assume a chunk size of 256K. That gives a total stripe width of 1024K. So, telling the lvm subsystem to align the start of the data on a 1024K offset will optimally align the lv on the pv. If you then create an ext4 filesystem on the lv, and tell the ext4 filesystem that you have a chunk size of 256k and a stripe width of 1024k, the ext4 filesystem will be properly aligned on the underlying raid device. And because you've told the ext4 filesystem about the raid device layouts, it will attempt to optimize access patterns for the raid device.

That all being said, here's an example of a non-optimal access pattern. Let's assume you have a 1024k write, and the ext4 filesystem knows you have a 1024k stripe width. The filesystem will attempt to align that write on a 1024k stripe boundary so that you get a full stripe write. That means that the raid layer will ignore the parity already on disk, will simply calculate new parity by doing an xor on the 1024k of data, and will then simply write all 4 256k chunks and the 256k parity block out to disk. That's optimal. If the alignment is skewed by the lvm layer though, what happens is that the ext4 filesystems tries to lay out the write on the start of a stripe but fails, and instead of the write causing a very fast parity generation and write to a single stripe, the write gets split between two different stripes and since neither stripe is a full stripe write, we do one of two things: a read-modify-write cycle or a read-calculate- write cycle. In either of those cases, it is a requirement that we read something off of disk and use it in the calculation of what needs to be written out to disk. So, we end up touching two stripes instead of one and we have to read stuff in, introducing a latency delay, before we can write our data out. So, it's highly important that, in so far as some layers are aware of raid device layouts, that those layers be *properly* aligned on our raid device or the result is not only suboptimal, but likely pathological.

 The only conceivable impact on performance
would be at some 'boundary' point -- if your volume contained
multiple physical partitions -- but those would be far and few between large areas where it should (?) make no difference. Eh?

Linda







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