linux-kernel - [RFC][PATCH] block: enable by default support for large devices and files on 32-bit archs

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Date:	Sat, 28 Mar 2009 19:16:06 +0100
From:	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	"Theodore Ts'o" <tytso@....edu>, linux-kernel@...r.kernel.org
Subject: [RFC][PATCH] block: enable by default support for large devices and files on 32-bit archs


Enable by default support for large devices and files (CONFIG_LBD):

- With 1TB disks being a commodity hardware it is quite easy to hit 2TB
  limitation while building RAIDs etc. and many distros have been using
  CONFIG_LBD=y by default already (at least Fedora 10 and openSUSE 11.1).

- This should also prevent a subtle ext4 filesystem compatibility issue:
  mke2fs.ext4 defaults to creating filesystems with huge_files feature
  enabled and such filesystems cannot be later mounted read-write on
  machines with CONFIG_LBD=n (it should be quite easy to hit this issue
  when trying to use filesystem created using distro kernel on system
  running the self-build kernel, think about USB disk enclosures & co.).

While at it:

- Clarify config option help text w.r.t. mounting ext4 filesystems
  (they can be mounted with CONFIG_LBD=n but in the read-only mode).

Cc: "Theodore Ts'o" <tytso@....edu>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@...il.com>
---
Unfortunately to also update the existings [def]configs the config option
rename would be needed.  Actually, since the performance impact and kernel
size increase seem to be minimal (at least on x86-32) maybe we should just
go ahead and remove CONFIG_LBD (== always use 64-bit sector_t/blkcnt_t)?

I can make needed changes once there is agreement on preferred approach...

 block/Kconfig |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

Index: b/block/Kconfig
===================================================================
--- a/block/Kconfig
+++ b/block/Kconfig
@@ -26,6 +26,7 @@ if BLOCK
 config LBD
 	bool "Support for large block devices and files"
 	depends on !64BIT
+	default y
 	help
 	  Enable block devices or files of size 2TB and larger.
 
@@ -38,11 +39,13 @@ config LBD
 
 	  The ext4 filesystem requires that this feature be enabled in
 	  order to support filesystems that have the huge_file feature
-	  enabled.    Otherwise, it will refuse to mount any filesystems
-	  that use the huge_file feature, which is enabled by default
-	  by mke2fs.ext4.   The GFS2 filesystem also requires this feature.
+	  enabled.  Otherwise, it will refuse to mount in the read-write
+	  mode any filesystems that use the huge_file feature, which is
+	  enabled by default by mke2fs.ext4.
 
-	  If unsure, say N.
+	  The GFS2 filesystem also requires this feature.
+
+	  If unsure, say Y.
 
 config BLK_DEV_IO_TRACE
 	bool "Support for tracing block io actions"
--
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