* [PATCH 1/2] Btrfs: add datacow flag in inode flag @ 2011-03-03 8:35 liubo 2011-03-15 20:26 ` Chris Mason 0 siblings, 1 reply; 9+ messages in thread From: liubo @ 2011-03-03 8:35 UTC (permalink / raw) To: Linux Btrfs; +Cc: linux-fsdevel For datacow control, the corresponding inode flags are needed. This is for the following patch. Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com> --- include/linux/fs.h | 2 ++ 1 files changed, 2 insertions(+), 0 deletions(-) diff --git a/include/linux/fs.h b/include/linux/fs.h index 63d069b..bef47ff 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -353,6 +353,8 @@ struct inodes_stat_t { #define FS_TOPDIR_FL 0x00020000 /* Top of directory hierarchies*/ #define FS_EXTENT_FL 0x00080000 /* Extents */ #define FS_DIRECTIO_FL 0x00100000 /* Use direct i/o */ +#define FS_NOCOW_FL 0x00800000 /* Do not cow file */ +#define FS_COW_FL 0x01000000 /* Cow file */ #define FS_RESERVED_FL 0x80000000 /* reserved for ext2 lib */ #define FS_FL_USER_VISIBLE 0x0003DFFF /* User visible flags */ -- 1.6.5.2 ^ permalink raw reply related [flat|nested] 9+ messages in thread
* Re: [PATCH 1/2] Btrfs: add datacow flag in inode flag 2011-03-03 8:35 [PATCH 1/2] Btrfs: add datacow flag in inode flag liubo @ 2011-03-15 20:26 ` Chris Mason 2011-03-15 20:57 ` Christoph Hellwig 0 siblings, 1 reply; 9+ messages in thread From: Chris Mason @ 2011-03-15 20:26 UTC (permalink / raw) To: liubo; +Cc: Linux Btrfs, linux-fsdevel, hch, Theodore Tso Excerpts from liubo's message of 2011-03-03 03:35:42 -0500: > > For datacow control, the corresponding inode flags are needed. > This is for the following patch. > > Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com> > --- > include/linux/fs.h | 2 ++ > 1 files changed, 2 insertions(+), 0 deletions(-) > > diff --git a/include/linux/fs.h b/include/linux/fs.h > index 63d069b..bef47ff 100644 > --- a/include/linux/fs.h > +++ b/include/linux/fs.h > @@ -353,6 +353,8 @@ struct inodes_stat_t { > #define FS_TOPDIR_FL 0x00020000 /* Top of directory hierarchies*/ > #define FS_EXTENT_FL 0x00080000 /* Extents */ > #define FS_DIRECTIO_FL 0x00100000 /* Use direct i/o */ > +#define FS_NOCOW_FL 0x00800000 /* Do not cow file */ > +#define FS_COW_FL 0x01000000 /* Cow file */ > #define FS_RESERVED_FL 0x80000000 /* reserved for ext2 lib */ > > #define FS_FL_USER_VISIBLE 0x0003DFFF /* User visible flags */ Hi everyone, I'd like to go ahead and include these for btrfs to use. Are there objections or a different preferred interface? -chris ^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH 1/2] Btrfs: add datacow flag in inode flag 2011-03-15 20:26 ` Chris Mason @ 2011-03-15 20:57 ` Christoph Hellwig 2011-03-15 22:06 ` Andreas Dilger 0 siblings, 1 reply; 9+ messages in thread From: Christoph Hellwig @ 2011-03-15 20:57 UTC (permalink / raw) To: Chris Mason; +Cc: liubo, Linux Btrfs, linux-fsdevel, hch, Theodore Tso On Tue, Mar 15, 2011 at 04:26:50PM -0400, Chris Mason wrote: > Excerpts from liubo's message of 2011-03-03 03:35:42 -0500: I'm fine with it. I'll defer the check for conflicts with extN-specific flags to Ted, though. ^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH 1/2] Btrfs: add datacow flag in inode flag 2011-03-15 20:57 ` Christoph Hellwig @ 2011-03-15 22:06 ` Andreas Dilger 2011-03-15 23:35 ` Chris Mason 0 siblings, 1 reply; 9+ messages in thread From: Andreas Dilger @ 2011-03-15 22:06 UTC (permalink / raw) To: Christoph Hellwig, Amir Goldstein Cc: Chris Mason, liubo, Linux Btrfs, linux-fsdevel, Theodore Tso On 2011-03-15, at 2:57 PM, Christoph Hellwig wrote: > On Tue, Mar 15, 2011 at 04:26:50PM -0400, Chris Mason wrote: >> #define FS_EXTENT_FL 0x00080000 /* Extents */ >> #define FS_DIRECTIO_FL 0x00100000 /* Use direct i/o */ >> +#define FS_NOCOW_FL 0x00800000 /* Do not cow file */ >> +#define FS_COW_FL 0x01000000 /* Cow file */ >> #define FS_RESERVED_FL 0x80000000 /* reserved for ext2 lib */ > > I'm fine with it. I'll defer the check for conflicts with extN-specific flags > to Ted, though. Looking at the upstream e2fsprogs I see in that range: > #define EXT4_EXTENTS_FL 0x00080000 /* Inode uses extents */ > #define EXT4_EA_INODE_FL 0x00200000 /* Inode used for large EA */ > #define EXT4_EOFBLOCKS_FL 0x00400000 /* Blocks allocated beyond EOF */ > #define EXT4_SNAPFILE_FL 0x01000000 /* Inode is a snapshot */ > #define EXT4_SNAPFILE_DELETED_FL 0x04000000 /* Snapshot is being deleted */ > #define EXT4_SNAPFILE_SHRUNK_FL 0x08000000 /* Snapshot shrink has completed */ > #define EXT2_RESERVED_FL 0x80000000 /* reserved for ext2 lib */ > > #define EXT2_FL_USER_VISIBLE 0x004BDFFF /* User visible flags */ so there is a conflict with FS_COW_FL and EXT4_SNAPFILE_FL. I don't know the semantics of those two flags enough to say for sure whether it is reasonable that they alias to each other, but at first glance "COW" and "SNAPSHOT" don't seem completely unrelated. It also isn't clear to me whether the SNAPFILE_DELETED_FL and SNAPFILE_SHRUNK_FL really need to be persistent flags that are stored on disk, or if they are state flags that should only be stored in the in-memory inode? Not having them in the limited remaining inode flag space would be better... Amir? Cheers, Andreas ^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH 1/2] Btrfs: add datacow flag in inode flag 2011-03-15 22:06 ` Andreas Dilger @ 2011-03-15 23:35 ` Chris Mason 2011-03-16 9:06 ` Amir Goldstein 0 siblings, 1 reply; 9+ messages in thread From: Chris Mason @ 2011-03-15 23:35 UTC (permalink / raw) To: Andreas Dilger Cc: Christoph Hellwig, Amir Goldstein, liubo, Linux Btrfs, linux-fsdevel, Theodore Tso Excerpts from Andreas Dilger's message of 2011-03-15 18:06:49 -0400: > On 2011-03-15, at 2:57 PM, Christoph Hellwig wrote: > > On Tue, Mar 15, 2011 at 04:26:50PM -0400, Chris Mason wrote: > >> #define FS_EXTENT_FL 0x00080000 /* Extents */ > >> #define FS_DIRECTIO_FL 0x00100000 /* Use direct i/o */ > >> +#define FS_NOCOW_FL 0x00800000 /* Do not cow file */ > >> +#define FS_COW_FL 0x01000000 /* Cow file */ > >> #define FS_RESERVED_FL 0x80000000 /* reserved for ext2 lib */ > > > > I'm fine with it. I'll defer the check for conflicts with extN-specific flags > > to Ted, though. > > Looking at the upstream e2fsprogs I see in that range: > > > #define EXT4_EXTENTS_FL 0x00080000 /* Inode uses extents */ > > #define EXT4_EA_INODE_FL 0x00200000 /* Inode used for large EA */ > > #define EXT4_EOFBLOCKS_FL 0x00400000 /* Blocks allocated beyond EOF */ > > #define EXT4_SNAPFILE_FL 0x01000000 /* Inode is a snapshot */ > > #define EXT4_SNAPFILE_DELETED_FL 0x04000000 /* Snapshot is being deleted */ > > #define EXT4_SNAPFILE_SHRUNK_FL 0x08000000 /* Snapshot shrink has completed */ > > #define EXT2_RESERVED_FL 0x80000000 /* reserved for ext2 lib */ > > > > #define EXT2_FL_USER_VISIBLE 0x004BDFFF /* User visible flags */ > > so there is a conflict with FS_COW_FL and EXT4_SNAPFILE_FL. I don't know the semantics of those two flags enough to say for sure whether it is reasonable that they alias to each other, but at first glance "COW" and "SNAPSHOT" don't seem completely unrelated. In the btrfs case FS_COW_FL means to do COW even when there are no snapshots. FS_NOCOW_FL means to do cow only when there are snapshots. -chris ^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH 1/2] Btrfs: add datacow flag in inode flag 2011-03-15 23:35 ` Chris Mason @ 2011-03-16 9:06 ` Amir Goldstein 2011-03-17 2:10 ` liubo 0 siblings, 1 reply; 9+ messages in thread From: Amir Goldstein @ 2011-03-16 9:06 UTC (permalink / raw) To: Chris Mason Cc: Andreas Dilger, Christoph Hellwig, liubo, Linux Btrfs, linux-fsdevel, Theodore Tso On Wed, Mar 16, 2011 at 1:35 AM, Chris Mason <chris.mason@oracle.com> wrote: > Excerpts from Andreas Dilger's message of 2011-03-15 18:06:49 -0400: >> On 2011-03-15, at 2:57 PM, Christoph Hellwig wrote: >> > On Tue, Mar 15, 2011 at 04:26:50PM -0400, Chris Mason wrote: >> >> #define FS_EXTENT_FL 0x00080000 /* Extents */ >> >> #define FS_DIRECTIO_FL 0x00100000 /* Use direct i/o */ >> >> +#define FS_NOCOW_FL 0x00800000 /* Do not cow file */ >> >> +#define FS_COW_FL 0x01000000 /* Cow file */ >> >> #define FS_RESERVED_FL 0x80000000 /* reserved for ext2 lib */ >> > >> > I'm fine with it. I'll defer the check for conflicts with extN-specific flags >> > to Ted, though. >> >> Looking at the upstream e2fsprogs I see in that range: >> >> > #define EXT4_EXTENTS_FL 0x00080000 /* Inode uses extents */ >> > #define EXT4_EA_INODE_FL 0x00200000 /* Inode used for large EA */ >> > #define EXT4_EOFBLOCKS_FL 0x00400000 /* Blocks allocated beyond EOF */ >> > #define EXT4_SNAPFILE_FL 0x01000000 /* Inode is a snapshot */ >> > #define EXT4_SNAPFILE_DELETED_FL 0x04000000 /* Snapshot is being deleted */ >> > #define EXT4_SNAPFILE_SHRUNK_FL 0x08000000 /* Snapshot shrink has completed */ >> > #define EXT2_RESERVED_FL 0x80000000 /* reserved for ext2 lib */ >> > >> > #define EXT2_FL_USER_VISIBLE 0x004BDFFF /* User visible flags */ >> >> so there is a conflict with FS_COW_FL and EXT4_SNAPFILE_FL. I don't know the semantics of those two flags enough to say for sure whether it is reasonable that they alias to each other, but at first glance "COW" and "SNAPSHOT" don't seem completely unrelated. EXT4_SNAPFILE_FL indicates a special system snapshot file, so it has no equivalence relation with FS_COW_FL. Please use 0x02000000 for FS_COW_FL. EXT4_SNAPFILE_DELETED_FL is a persistent state of a snapshot file, which is no longer available as a mountable device, but cannot be unlinked because it holds changed data sets needed by older snapshots. EXT4_SNAPFILE_SHRUNK_FL is a persistent state of a (deleted) snapshot file, which has undergone a "shrink" process to free all change sets not needed by older snapshots. The persistence of the flag is needed to avoid tedious shrinking when it is not needed. > > In the btrfs case FS_COW_FL means to do COW even when there are no > snapshots. FS_NOCOW_FL means to do cow only when there are snapshots. > I am interested in FS_NOCOW_FL as well, but for my implementation it would mean do not do COW on rewrites even when there are snapshots, so a user can create a pre-allocated "island of blocks", which are pinned to a physical location, for raw VM image for example. Thanks, Amir. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html ^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH 1/2] Btrfs: add datacow flag in inode flag 2011-03-16 9:06 ` Amir Goldstein @ 2011-03-17 2:10 ` liubo 2011-03-17 14:21 ` Chris Mason 0 siblings, 1 reply; 9+ messages in thread From: liubo @ 2011-03-17 2:10 UTC (permalink / raw) To: Amir Goldstein Cc: Chris Mason, Andreas Dilger, Christoph Hellwig, Linux Btrfs, linux-fsdevel, Theodore Tso On 03/16/2011 05:06 PM, Amir Goldstein wrote: > On Wed, Mar 16, 2011 at 1:35 AM, Chris Mason <chris.mason@oracle.com> wrote: >> Excerpts from Andreas Dilger's message of 2011-03-15 18:06:49 -0400: >>> On 2011-03-15, at 2:57 PM, Christoph Hellwig wrote: >>>> On Tue, Mar 15, 2011 at 04:26:50PM -0400, Chris Mason wrote: >>>>> #define FS_EXTENT_FL 0x00080000 /* Extents */ >>>>> #define FS_DIRECTIO_FL 0x00100000 /* Use direct i/o */ >>>>> +#define FS_NOCOW_FL 0x00800000 /* Do not cow file */ >>>>> +#define FS_COW_FL 0x01000000 /* Cow file */ >>>>> #define FS_RESERVED_FL 0x80000000 /* reserved for ext2 lib */ >>>> I'm fine with it. I'll defer the check for conflicts with extN-specific flags >>>> to Ted, though. >>> Looking at the upstream e2fsprogs I see in that range: >>> >>>> #define EXT4_EXTENTS_FL 0x00080000 /* Inode uses extents */ >>>> #define EXT4_EA_INODE_FL 0x00200000 /* Inode used for large EA */ >>>> #define EXT4_EOFBLOCKS_FL 0x00400000 /* Blocks allocated beyond EOF */ >>>> #define EXT4_SNAPFILE_FL 0x01000000 /* Inode is a snapshot */ >>>> #define EXT4_SNAPFILE_DELETED_FL 0x04000000 /* Snapshot is being deleted */ >>>> #define EXT4_SNAPFILE_SHRUNK_FL 0x08000000 /* Snapshot shrink has completed */ >>>> #define EXT2_RESERVED_FL 0x80000000 /* reserved for ext2 lib */ >>>> >>>> #define EXT2_FL_USER_VISIBLE 0x004BDFFF /* User visible flags */ >>> so there is a conflict with FS_COW_FL and EXT4_SNAPFILE_FL. I don't know the semantics of those two flags enough to say for sure whether it is reasonable that they alias to each other, but at first glance "COW" and "SNAPSHOT" don't seem completely unrelated. > > EXT4_SNAPFILE_FL indicates a special system snapshot file, so it has > no equivalence relation with FS_COW_FL. > Please use 0x02000000 for FS_COW_FL. Fine with that, but it's up to Chris. :) thanks, liubo > > EXT4_SNAPFILE_DELETED_FL is a persistent state of a snapshot file, > which is no longer > available as a mountable device, but cannot be unlinked because it > holds changed data sets > needed by older snapshots. > > EXT4_SNAPFILE_SHRUNK_FL is a persistent state of a (deleted) snapshot > file, which has > undergone a "shrink" process to free all change sets not needed by > older snapshots. > The persistence of the flag is needed to avoid tedious shrinking when > it is not needed. > > >> In the btrfs case FS_COW_FL means to do COW even when there are no >> snapshots. FS_NOCOW_FL means to do cow only when there are snapshots. >> > > I am interested in FS_NOCOW_FL as well, but for my implementation it would mean > do not do COW on rewrites even when there are snapshots, so a user can > create a pre-allocated > "island of blocks", which are pinned to a physical location, for raw > VM image for example. > > > Thanks, > Amir. > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > ^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH 1/2] Btrfs: add datacow flag in inode flag 2011-03-17 2:10 ` liubo @ 2011-03-17 14:21 ` Chris Mason 2011-03-17 14:37 ` Amir Goldstein 0 siblings, 1 reply; 9+ messages in thread From: Chris Mason @ 2011-03-17 14:21 UTC (permalink / raw) To: liubo Cc: Amir Goldstein, Andreas Dilger, Christoph Hellwig, Linux Btrfs, linux-fsdevel, Theodore Tso Excerpts from liubo's message of 2011-03-16 22:10:09 -0400: > On 03/16/2011 05:06 PM, Amir Goldstein wrote: > > On Wed, Mar 16, 2011 at 1:35 AM, Chris Mason <chris.mason@oracle.com> wrote: > >> Excerpts from Andreas Dilger's message of 2011-03-15 18:06:49 -0400: > >>> On 2011-03-15, at 2:57 PM, Christoph Hellwig wrote: > >>>> On Tue, Mar 15, 2011 at 04:26:50PM -0400, Chris Mason wrote: > >>>>> #define FS_EXTENT_FL 0x00080000 /* Extents */ > >>>>> #define FS_DIRECTIO_FL 0x00100000 /* Use direct i/o */ > >>>>> +#define FS_NOCOW_FL 0x00800000 /* Do not cow file */ > >>>>> +#define FS_COW_FL 0x01000000 /* Cow file */ > >>>>> #define FS_RESERVED_FL 0x80000000 /* reserved for ext2 lib */ > >>>> I'm fine with it. I'll defer the check for conflicts with extN-specific flags > >>>> to Ted, though. > >>> Looking at the upstream e2fsprogs I see in that range: > >>> > >>>> #define EXT4_EXTENTS_FL 0x00080000 /* Inode uses extents */ > >>>> #define EXT4_EA_INODE_FL 0x00200000 /* Inode used for large EA */ > >>>> #define EXT4_EOFBLOCKS_FL 0x00400000 /* Blocks allocated beyond EOF */ > >>>> #define EXT4_SNAPFILE_FL 0x01000000 /* Inode is a snapshot */ > >>>> #define EXT4_SNAPFILE_DELETED_FL 0x04000000 /* Snapshot is being deleted */ > >>>> #define EXT4_SNAPFILE_SHRUNK_FL 0x08000000 /* Snapshot shrink has completed */ > >>>> #define EXT2_RESERVED_FL 0x80000000 /* reserved for ext2 lib */ > >>>> > >>>> #define EXT2_FL_USER_VISIBLE 0x004BDFFF /* User visible flags */ > >>> so there is a conflict with FS_COW_FL and EXT4_SNAPFILE_FL. I don't know the semantics of those two flags enough to say for sure whether it is reasonable that they alias to each other, but at first glance "COW" and "SNAPSHOT" don't seem completely unrelated. > > > > EXT4_SNAPFILE_FL indicates a special system snapshot file, so it has > > no equivalence relation with FS_COW_FL. > > Please use 0x02000000 for FS_COW_FL. > > Fine with that, but it's up to Chris. :) I'd rather not conflict unless we're critically short on space. > > > > EXT4_SNAPFILE_DELETED_FL is a persistent state of a snapshot file, > > which is no longer > > available as a mountable device, but cannot be unlinked because it > > holds changed data sets > > needed by older snapshots. > > > > EXT4_SNAPFILE_SHRUNK_FL is a persistent state of a (deleted) snapshot > > file, which has > > undergone a "shrink" process to free all change sets not needed by > > older snapshots. > > The persistence of the flag is needed to avoid tedious shrinking when > > it is not needed. > > > > > >> In the btrfs case FS_COW_FL means to do COW even when there are no > >> snapshots. FS_NOCOW_FL means to do cow only when there are snapshots. > >> > > > > I am interested in FS_NOCOW_FL as well, but for my implementation it would mean > > do not do COW on rewrites even when there are snapshots, so a user can > > create a pre-allocated > > "island of blocks", which are pinned to a physical location, for raw > > VM image for example. I'm not sure how the island of blocks idea can work with snapshots? Wouldn't the snapshot corrupt if anything in the island were changed? -chris ^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH 1/2] Btrfs: add datacow flag in inode flag 2011-03-17 14:21 ` Chris Mason @ 2011-03-17 14:37 ` Amir Goldstein 0 siblings, 0 replies; 9+ messages in thread From: Amir Goldstein @ 2011-03-17 14:37 UTC (permalink / raw) To: Chris Mason Cc: liubo, Andreas Dilger, Christoph Hellwig, Linux Btrfs, linux-fsdevel, Theodore Tso On Thu, Mar 17, 2011 at 4:21 PM, Chris Mason <chris.mason@oracle.com> wrote: > Excerpts from liubo's message of 2011-03-16 22:10:09 -0400: >> On 03/16/2011 05:06 PM, Amir Goldstein wrote: >> > On Wed, Mar 16, 2011 at 1:35 AM, Chris Mason <chris.mason@oracle.com> wrote: >> >> Excerpts from Andreas Dilger's message of 2011-03-15 18:06:49 -0400: >> >>> On 2011-03-15, at 2:57 PM, Christoph Hellwig wrote: >> >>>> On Tue, Mar 15, 2011 at 04:26:50PM -0400, Chris Mason wrote: >> >>>>> #define FS_EXTENT_FL 0x00080000 /* Extents */ >> >>>>> #define FS_DIRECTIO_FL 0x00100000 /* Use direct i/o */ >> >>>>> +#define FS_NOCOW_FL 0x00800000 /* Do not cow file */ >> >>>>> +#define FS_COW_FL 0x01000000 /* Cow file */ >> >>>>> #define FS_RESERVED_FL 0x80000000 /* reserved for ext2 lib */ >> >>>> I'm fine with it. I'll defer the check for conflicts with extN-specific flags >> >>>> to Ted, though. >> >>> Looking at the upstream e2fsprogs I see in that range: >> >>> >> >>>> #define EXT4_EXTENTS_FL 0x00080000 /* Inode uses extents */ >> >>>> #define EXT4_EA_INODE_FL 0x00200000 /* Inode used for large EA */ >> >>>> #define EXT4_EOFBLOCKS_FL 0x00400000 /* Blocks allocated beyond EOF */ >> >>>> #define EXT4_SNAPFILE_FL 0x01000000 /* Inode is a snapshot */ >> >>>> #define EXT4_SNAPFILE_DELETED_FL 0x04000000 /* Snapshot is being deleted */ >> >>>> #define EXT4_SNAPFILE_SHRUNK_FL 0x08000000 /* Snapshot shrink has completed */ >> >>>> #define EXT2_RESERVED_FL 0x80000000 /* reserved for ext2 lib */ >> >>>> >> >>>> #define EXT2_FL_USER_VISIBLE 0x004BDFFF /* User visible flags */ >> >>> so there is a conflict with FS_COW_FL and EXT4_SNAPFILE_FL. I don't know the semantics of those two flags enough to say for sure whether it is reasonable that they alias to each other, but at first glance "COW" and "SNAPSHOT" don't seem completely unrelated. >> > >> > EXT4_SNAPFILE_FL indicates a special system snapshot file, so it has >> > no equivalence relation with FS_COW_FL. >> > Please use 0x02000000 for FS_COW_FL. >> >> Fine with that, but it's up to Chris. :) > > I'd rather not conflict unless we're critically short on space. > >> > >> > EXT4_SNAPFILE_DELETED_FL is a persistent state of a snapshot file, >> > which is no longer >> > available as a mountable device, but cannot be unlinked because it >> > holds changed data sets >> > needed by older snapshots. >> > >> > EXT4_SNAPFILE_SHRUNK_FL is a persistent state of a (deleted) snapshot >> > file, which has >> > undergone a "shrink" process to free all change sets not needed by >> > older snapshots. >> > The persistence of the flag is needed to avoid tedious shrinking when >> > it is not needed. >> > >> > >> >> In the btrfs case FS_COW_FL means to do COW even when there are no >> >> snapshots. FS_NOCOW_FL means to do cow only when there are snapshots. >> >> >> > >> > I am interested in FS_NOCOW_FL as well, but for my implementation it would mean >> > do not do COW on rewrites even when there are snapshots, so a user can >> > create a pre-allocated >> > "island of blocks", which are pinned to a physical location, for raw >> > VM image for example. > > I'm not sure how the island of blocks idea can work with snapshots? > Wouldn't the snapshot corrupt if anything in the island were changed? > It would corrupt, but only to the extent that the file to which you requested NOCOW may contain newer data. It cannot contain uninitialized data, because truncating the file would leave it's blocks referenced by the snapshot. Think of a large database file, which is already replicated and hot backed up regularly. An arbitrary snapshot of that file will give you a copy for disaster recovery at best. Not sure this is worth the effort of COWing it and fragmenting it beyond recognition. Amir. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html ^ permalink raw reply [flat|nested] 9+ messages in thread
end of thread, other threads:[~2011-03-17 14:37 UTC | newest] Thread overview: 9+ messages (download: mbox.gz follow: Atom feed -- links below jump to the message on this page -- 2011-03-03 8:35 [PATCH 1/2] Btrfs: add datacow flag in inode flag liubo 2011-03-15 20:26 ` Chris Mason 2011-03-15 20:57 ` Christoph Hellwig 2011-03-15 22:06 ` Andreas Dilger 2011-03-15 23:35 ` Chris Mason 2011-03-16 9:06 ` Amir Goldstein 2011-03-17 2:10 ` liubo 2011-03-17 14:21 ` Chris Mason 2011-03-17 14:37 ` Amir Goldstein
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