Bug 230803

The comment is correct. It has been an oversight of GFS implementation.

A quick fix might be in fill_super(), right after GFS registers the diaper 
device, the readahead value can be pulled from the ra_pages of backing_dev_info. 
       

I'll try to get the test patch done today. 

well, after reading the source code carefully, I see diaper device does
get its backing_dev_info from the real device (in diaper_get()):

    363         diaper = bdget_disk(gd, 0);
    364         if (!diaper)
    365                 goto fail_remove;
    366
    367         down(&diaper->bd_sem);
    368         if (!diaper->bd_openers) {
    369                 diaper->bd_disk = gd;
    370                 diaper->bd_contains = diaper;
    371                 bd_set_size(diaper, 0x7FFFFFFFFFFFFFFFULL);
    372                 diaper->bd_inode->i_data.backing_dev_info = &gd->queue->
       backing_dev_info;
    373         } else
    374                 printk("GFS: diaper: reopening\n");

This is further confirmed by tracing from vfs super block in the entry 
of gfs_write, into s_bdev->bd_inode->i_mapping by examining the run time 
kernel memory:

crash> struct backing_dev_info 0x10119b1e6e0
struct backing_dev_info {
  ra_pages = 32,
  state = 0,
  memory_backed = 0,
  congested_fn = 0xffffffffa033beb0,
  congested_data = 0x1007dbcda80,
  unplug_io_fn = 0xffffffff8024e2c2 <blk_backing_dev_unplug>,
  unplug_io_data = 0x10119b1e490
}

So GFS doesn't miss any readahead. However, 32 may be good for general 
purpose readahead, not for large sequential write ? 

We happen to have a customer complaning about large sequential read
performance. Will give out a tunable to see whether it could improve
the number. If proved useful, we'll take it for RHEL 4.5

Or, depending on the device, the driver may not set readahead for its queue
at all. So a tunable would be even more proper. 

VFS layer happens to offer a backing device info block in its super block
structure to allow filesystem to override the default setting of its under-
lying block device. 

After adding this function into GFS as a tunable (seq_readahead), a quick 
and dirty dd test with 2G file shows the following result on my test 
cluster:

default dd read: 27.76s 
2048 readahead: 15.19s.

Will continue this testing.

Created attachment 150135 [details]
Tentative patch for sequential read performance fix

The above patch is not finalized yet - needs some clean up but is good for
test runs. 

ok, after few loops, it is un-mistakenly 2x performance gain. Mathieu, thanks
for bringing this issue up !

Moving to proposed for 4.6 - too late in the cycle for 4.5

Our TAM just tested this patch out 

Sorry, hit submit key too soon..

This patch seems to significantly help IPTV read performance. The tunable is 
set by "gfs_tool settune <mount> seq_readahead <val>" command. The <val> we 
use is subject to the file size. Assuming 2G file with 2048 readahead is good,
in IPTV test runs, with file size between 400-500M, we use 256 readahead. The 
change made us passing ext3 number and TAM seems to be very pleased with the 
results.  

Code checked into CVS. Move bugzilla into modified state. 

An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on the solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.

http://rhn.redhat.com/errata/RHBA-2007-0998.html