RE: Big I/O requests are split into small ones due to unaligned ext4 partition boundary?

[Dexuan Cui]

> From: Ming Lei [mailto:tom.leiming@xxxxxxxxx]
> Sent: Thursday, December 15, 2016 20:43
> 
> On Thu, Dec 15, 2016 at 7:47 PM, Dexuan Cui <decui@xxxxxxxxxxxxx> wrote:
> > Hi, when I run "mkfs.ext4 /dev/sdc2" in a Linux virtual machine on Hyper-V,
> > where a disk IOPS=500 limit is applied by me [0],  the command takes much
> > more time, if the ext4 partition boundary is not properly aligned:
> >
> > Example 1 [1]: it takes ~7 minutes with average wMB/s = 0.3   (slow)
> > Example 2 [2]: it takes ~3.5 minutes with average wMB/s = 0.6 (slow)
> > Example 3 [3]: it takes ~0.5 minute with average wMB/s = 4 (expected)
> >
> > strace shows the mkfs.ext3 program calls seek()/write() a lot and most of
> > the writes use 32KB buffers (this should be big enough), and the program
> > only invokes fsync() once, after it issues all the writes -- the fsync() takes
> >>99% of the time.
> >
> > By logging SCSI commands, the SCSI Write(10) command is used here for the
> > userspace 32KB write:
> > in example 1, *each* command writes 1 or 2 sectors only (1 sector = 512
> bytes);
> > in example 2, *each* command writes 2 or 4 sectors only;
> > in example 3, each command writes 1024 sectors.
> >
> > It looks the kernel block I/O layer can somehow split big user-space buffers
> > into really small write requests (1, 2, and 4 sectors)?
> > This looks really strange to me.
> >
> > Note: in my test, this strange issue happens to 4.4 and the mainline 4.9 kernels,
> > but the stable 3.18.45 kernel doesn't have the issue, i.e. all the 3 above test
> > examples can finish in ~0.5 minute.
> >
> > Any comment?
> 
> I remember that we discussed this kind of issue, please see the discussion[1]
> and check if the patch[2] can fix your issue.
> 
> [1] http://marc.info/?t=145805525500002&r=1&w=2
> [2] http://marc.info/?l=linux-kernel&m=145934325429152&w=2
> 
> Ming
 
Thank you very much, Ming! The patch can fix my issue!
It looks your patch was not merged into the upstream somehow.
Would you please submit the patch again?

Thanks,
-- Dexuan