On 03/18/2015 02:55 PM, J. Bruce Fields wrote:
> On Wed, Mar 18, 2015 at 02:16:29PM -0400, Anna Schumaker wrote:
>> On 03/17/2015 05:36 PM, J. Bruce Fields wrote:
>>> On Tue, Mar 17, 2015 at 04:07:38PM -0400, J. Bruce Fields wrote:
>>>> On Tue, Mar 17, 2015 at 03:56:33PM -0400, J. Bruce Fields wrote:
>>>>> On Mon, Mar 16, 2015 at 05:18:08PM -0400, Anna Schumaker wrote:
>>>>>> This patch implements sending an array of segments back to the client.
>>>>>> Clients should be prepared to handle multiple segment reads to make this
>>>>>> useful. We try to splice the first data segment into the XDR result,
>>>>>> and remaining segments are encoded directly.
>>>>>
>>>>> I'm still interested in what would happen if we started with an
>>>>> implementation like:
>>>>>
>>>>> - if the entire requested range falls within a hole, return that
>>>>> single hole.
>>>>> - otherwise, just treat the thing as one big data segment.
>>>>>
>>>>> That would provide a benefit in the case there are large-ish holes
>>>>> with minimal impact otherwise.
>>>>>
>>>>> (Though patches for full support are still useful even if only for
>>>>> client-testing purposes.)
>>>>
>>>> Also, looks like
>>>>
>>>> xvs_io -c "fiemap -v" <file>
>>>>
>>>> will give hole sizes for a given <file>. (Thanks, esandeen.) Running
>>>> that on a few of my test vm images shows a fair number of large
>>>> (hundreds of megs) files, which suggests identifying only >=rwsize holes
>>>> might still be useful.
>>>
>>> Just for fun.... I wrote the following test program and ran it on my
>>> collection of testing vm's. Some looked like this:
>>>
>>> f21-1.qcow2
>>> 144784 -rw-------. 1 qemu qemu 8591507456 Mar 16 10:13 f21-1.qcow2
>>> total hole bytes: 8443252736 (98%)
>>> in aligned 1MB chunks: 8428453888 (98%)
>>>
>>> So, basically, read_plus would save transferring most of the data even
>>> when only handling 1MB holes.
>>>
>>> But some looked like this:
>>>
>>> 501524 -rw-------. 1 qemu qemu 8589934592 May 20 2014 rhel6-1-1.img
>>> total hole bytes: 8077516800 (94%)
>>> in aligned 1MB chunks: 0 (0%)
>>>
>>> So the READ_PLUS that caught every hole might save a lot, the one that
>>> only caught 1MB holes wouldn't help at all.
>>>
>>> And there were lots of examples in between those two extremes.
>>
>> I tested with three different 512 MB files: 100% data, 100% hole, and alternating every megabyte. The results were surprising:
>>
>> | v4.1 | v4.2
>> -----------------------
>> data | 0.685s | 0.714s
>> hole | 0.485s | 15.547s
>> mixed | 1.283s | 0.448
>>
>> >From what I can tell, the 100% hole case takes so long because of the
>>> SEEK_DATA call in nfsd4_encode_read_plus_hole(). I took this out to
>>> trick the function into thinking that the entire file was already a
>>> hole, and runtime dropped to the levels of v4.1 and v4.2.
>
> Wait, that 15s is due to just one SEEK_DATA?
The server is returning a larger hole than the client can read at once, so there are several SEEK_DATA calls made to verify that there are no data segments before the end of the file.
>
>> I wonder
>>> if this is filesystem dependent? My server is exporting ext4.
>
> Sounds like just a bug. I've been doing lots of lseek(.,.,SEEK_DATA) on
> both ext4 and xfs without seeing anything that weird.
It looks like something weird on ext4. I switched my exported filesystem to xfs:
| v4.1 | v4.2
------+--------+-------
data | 0.764s | 1.343s
hole | 0.572s | 0.205s
mixed | 0.634s | 0.472s
I bumped up the test to 1G files:
| v4.1 | v4.2
------+--------+-------
data | 1.578s | 1.743s
hole | 1.241s | 0.443s
mixed | 1.884s | 0.913s
Let me know if I should test anything larger!
Anna
>
> I believe it does return -ENXIO in the case SEEK_DATA is called at an
> offset beyond which there's no more data. At least that's what I saw in
> userspace. So maybe your code just isn't handling that case correctly?
>
> --b.
>
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