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[PATCH RFC] xfs: set block device logical sector size on xfs_buftarg

To: xfs-oss <xfs@xxxxxxxxxxx>
Subject: [PATCH RFC] xfs: set block device logical sector size on xfs_buftarg
From: Eric Sandeen <sandeen@xxxxxxxxxx>
Date: Wed, 13 Nov 2013 12:25:33 -0600
Delivered-to: xfs@xxxxxxxxxxx
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:24.0) Gecko/20100101 Thunderbird/24.1.0
Pure RFC; this might be crazy.  Here's the problem I'm trying to solve:

Today, mkfs.xfs will select a 4k sector size for a 4k physical / 512 logical
drive.  (that change was done by me).  The thought was that it'd be an
efficiency gain to not make the drive do the (possible) RMW cycles on
512-byte log IO, primarily.

However, now this restricts all DIO to 4k alignment, not the otherwise-
possible 512.

This came up when qemu-kvm, in cache=none mode, tries to boot off an
image hosted on such a filesystem, and its bios wants to do a 512 byte
direct IO read off the disk - it fails.

But I'm wondering - the buftarg's bt_sshift and bt_smask are only used
in a few places.  

XFS_IOC_DIOINFO - to get the minimum io size
xfs_file_aio_read() and xfs_file_dio_aio_write() to check alignment
_xfs_buf_find() to be sure we aren't doing sub-sector IO.

So what I'm wondering is:  Can we somehow separate the "sector size"
that i.e. primarily the xfs log does its IO in, based on sb_sectsize,
from the actual, hard-minimum possible IO, in the buftarg bt_sshift &
bt_smask.


Something like this, though untested, and I'm probably missing something.

Our other option, I guess, is to just revert the mkfs change which
picks the physical rather than logical sector size, and go back to 
512 if it's available as a logical size.

Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
---


diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 8968f50..58ce036 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -814,10 +814,12 @@ STATIC int
 xfs_setup_devices(
        struct xfs_mount        *mp)
 {
+       xfs_buftarg_t           btp;
        int                     error;
 
+       btp = mp->m_ddev_targp;
        error = xfs_setsize_buftarg(mp->m_ddev_targp, mp->m_sb.sb_blocksize,
-                                   mp->m_sb.sb_sectsize);
+                                   bdev_logical_block_size(btp));
        if (error)
                return error;
 
@@ -833,9 +835,10 @@ xfs_setup_devices(
                        return error;
        }
        if (mp->m_rtdev_targp) {
+               btp = mp->m_rtdev_targp;
                error = xfs_setsize_buftarg(mp->m_rtdev_targp,
                                            mp->m_sb.sb_blocksize,
-                                           mp->m_sb.sb_sectsize);
+                                           bdev_logical_block_size(btp));
                if (error)
                        return error;
        }


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