Re: [RESEND] [PATCH] readahead:add blk_run_backing_dev

Ronald Moesbergen
2009-07-16T14:54:47Z

2009/7/16 Vladislav Bolkhovitin <vst@vlnb.net>:
>
> Ronald Moesbergen, on 07/16/2009 11:32 AM wrote:
>>
>> 2009/7/15 Vladislav Bolkhovitin <vst@vlnb.net>:
>>>>
>>>> The drop with 64 max_sectors_kb on the client is a consequence of how
>>>> CFQ
>>>> is working. I can't find the exact code responsible for this, but from
>>>> all
>>>> signs, CFQ stops delaying requests if amount of outstanding requests
>>>> exceeds
>>>> some threshold, which is 2 or 3. With 64 max_sectors_kb and 5 SCST I/O
>>>> threads this threshold is exceeded, so CFQ doesn't recover order of
>>>> requests, hence the performance drop. With default 512 max_sectors_kb
>>>> and
>>>> 128K RA the server sees at max 2 requests at time.
>>>>
>>>> Ronald, can you perform the same tests with 1 and 2 SCST I/O threads,
>>>> please?
>>
>> Ok. Should I still use the file-on-xfs testcase for this, or should I
>> go back to using a regular block device?
>
> Yes, please

As in: Yes, go back to block device, or Yes use file-on-xfs?

>> The file-over-iscsi is quite
>> uncommon I suppose, most people will export a block device over iscsi,
>> not a file.
>
> No, files are common. The main reason why people use direct block devices is
> a not supported by anything believe that comparing with files they "have
> less overhead", so "should be faster". But it isn't true and can be easily
> checked.

Well, there are other advantages of using a block device: they are
generally more manageble, for instance you can use LVM for resizing
instead of strange dd magic to extend a file. When using a file you
have to extend the volume that holds the file first, and then the file
itself. And you don't lose disk space to filesystem metadata twice.
Also, I still don't get why reads/writes from a blockdevice are
different in speed than reads/writes from a file on a filesystem. I
for one will not be using files exported over iscsi, but blockdevices
(LVM volumes).

Ronald.