2009/7/16 Vladislav Bolkhovitin <vst@xxxxxxxx>: > > Ronald Moesbergen, on 07/16/2009 11:32 AM wrote: >> >> 2009/7/15 Vladislav Bolkhovitin <vst@xxxxxxxx>: >>>> >>>> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
