Yeah, snapshots was the other possibility and I should have looked closer in the code -- you are getting the default chunksize. You can specify the chunksize when you create the logical volumes below but as you said it's limited to 512K at the upper end. I briefly looked at some of the kernel code to try and understand this limitation. I'm just guessing but perhaps the 512K limit is because of the vmalloc in dm-exception-store.c? (maybe someone else on this list more familiar with this code can confirm and/or explain the limit) There is a simple piece of code in lvcreate.c that was changed around 9/2005 to impose the limitation. If you want to _experiment_ in a simpler way you might think about changing this, rebuilding lvm, and using the chunksize param to create your lvs. On Wed, 2006-11-22 at 14:25 +0800, Kalpak Shah wrote: > Hi, > > Thanks for your reply Dave. > > I am not using a striped target. The command I used for creating the > logical volume was: > > 1) lvcreate -nMDT -s500M volLustre > 2) lvcreate -nOST0 -s500M volLustre > > Then I created snapshot LVs for the above two LVs. > > 1) lvcreate -L200M -s -n MDTb1 /dev/volLustre/MDT > 2) lvcreate -L200M -s -n OST0b1 /dev/volLustre/OST0 > > I tried to give a chunksize of 1024 but the lvcreate command segfaults. > I googled and found that it works only uptil 512K(1024 sectors). > > Let me explain the problem in depth. Lustre sends writes of 1MB, which > are getting broken down (max size could be 512KB) into smaller sizes > reducing the overall throughput. In function max_io_len() I returned len > as it was without splitting it further. In that case the writes/reads > sent by Lustre do not get broken down and performance improves > considerably. Note that the reads/writes may be bigger in some cases. > > Thanks, > Kalpak. > > On Tue, 2006-11-21 at 13:02 -0500, Dave Wysochanski wrote: > > On Tue, 2006-11-21 at 19:20 +0800, Kalpak Shah wrote: > > > Hi,ate > > > > > > Hi I am checking if device-mapper breaks down the reads/writes which are > > > sent by the file system. For example if an FS like Lustre sends a 1MB > > > read/write whether the request goes to the disk as a single request or > > > is it broken down into many small requests. > > > > > It depends how you create the logical volume on which you put the > > filesystem. > > > > > I have put some printks in __map_bio, __split_bio, etc. It seems that > > > even though 1 MB writes are sent to the device mapper the bios that are > > > sent to generic_make_request in __map_bio are at max 8k in size. In > > > function max_io_len(), ti->split_io is just 16 sectors at max, hich is > > > why the requests get broken down. > > > > > > Why is this ->split_io variable set to 16 sectors? Can it be more (arnd > > > 2048 sectors) without affecting the stability? > > > > > > > That sounds like you created a logical volume with stripe size (in > > device mapper it's termed "chunk size") of 8KB. > > What is the output of "dmsetup table" for the volume in question? > > > > The default stripe size is 64KB (128 sectors) but you can change it when > > you create the lv. An example on my setup I got this for one lv I > > created (notice the 16 in the output): > > # lvcreate -L 296M -i 2 --stripesize 8K -n lv0 vg0 > > Logical volume "lv0" created > > # dmsetup table > > vg0-lv0: 0 606208 striped 2 16 7:3 384 7:4 384 > > > > The format for the striped target is: > > /* > > * Construct a striped mapping. > > * <number of stripes> <chunk size (2^^n)> [<dev_path> <offset>]+ > > */ > > > > > > -- > > dm-devel mailing list > > dm-devel@xxxxxxxxxx > > https://www.redhat.com/mailman/listinfo/dm-devel > > > > > -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
