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Date: Wed, 1 Apr 2015 23:06:11 -0500
From: Andreas Dilger <adilger@...ger.ca>
To: "Darrick J. Wong" <darrick.wong@...cle.com>
Cc: "tytso@....edu" <tytso@....edu>,
"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 10/35] undo-io: add new calls to and speed up the undo io manager
Doesn't it kind of make e2undo useless if it doesn't work unless
the overwriting operation completed successfully?
Wouldn't it be better to save the superblock at the start, so that
it is available if the overwriting operation is interrupted? It seems
like e2undo would be most useful if e.g. resize2fs was interrupted in
the middle of some otherwise-corrupting change to the
filesystem.
While speeding up undo logging is nice, being able to recover your
filesystem in case of a problem is the primary goal, and that shouldn't be
forgotten. Otherwise you may as well not use the undo manager at all.
Cheers, Andreas
> On Apr 1, 2015, at 21:35, Darrick J. Wong <darrick.wong@...cle.com> wrote:
>
> Implement pass-through calls for discard, zero-out, and readahead in
> the IO manager so that we can take advantage of any underlying
> support.
>
> Furthermore, improve tdb write-out speed by disabling locking and only
> fsyncing at the end -- we don't care about locking because having
> multiple writers to the undo file will produce an undo database full
> of garbage blocks; and we only need to fsync at the end because if we
> fail before the end, our undo file will lack the necessary superblock
> data that e2undo requires to do replay safely. Without this, we call
> fsync four times per tdb update(!) This reduces the overhead of using
> undo_io while converting a 2TB FS to metadata_csum from 3+ hours to 55
> minutes.
>
> Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> ---
> lib/ext2fs/tdb.c | 10 ++++++
> lib/ext2fs/tdb.h | 2 +
> lib/ext2fs/undo_io.c | 87 +++++++++++++++++++++++++++++++++++++++++++++++++-
> 3 files changed, 97 insertions(+), 2 deletions(-)
>
>
> diff --git a/lib/ext2fs/tdb.c b/lib/ext2fs/tdb.c
> index 1d97685..7317288 100644
> --- a/lib/ext2fs/tdb.c
> +++ b/lib/ext2fs/tdb.c
> @@ -4142,3 +4142,13 @@ int tdb_reopen_all(int parent_longlived)
>
> return 0;
> }
> +
> +/**
> + * Flush a database file from the page cache.
> + **/
> +int tdb_flush(struct tdb_context *tdb)
> +{
> + if (tdb->fd != -1)
> + return fsync(tdb->fd);
> + return 0;
> +}
> diff --git a/lib/ext2fs/tdb.h b/lib/ext2fs/tdb.h
> index 732ef0e..6a4086c 100644
> --- a/lib/ext2fs/tdb.h
> +++ b/lib/ext2fs/tdb.h
> @@ -129,6 +129,7 @@ typedef struct TDB_DATA {
> #define tdb_lockall_nonblock ext2fs_tdb_lockall_nonblock
> #define tdb_lockall_read_nonblock ext2fs_tdb_lockall_read_nonblock
> #define tdb_lockall_unmark ext2fs_tdb_lockall_unmark
> +#define tdb_flush ext2fs_tdb_flush
>
> /* this is the context structure that is returned from a db open */
> typedef struct tdb_context TDB_CONTEXT;
> @@ -191,6 +192,7 @@ size_t tdb_map_size(struct tdb_context *tdb);
> int tdb_get_flags(struct tdb_context *tdb);
> void tdb_enable_seqnum(struct tdb_context *tdb);
> void tdb_increment_seqnum_nonblock(struct tdb_context *tdb);
> +int tdb_flush(struct tdb_context *tdb);
>
> /* Low level locking functions: use with care */
> int tdb_chainlock(struct tdb_context *tdb, TDB_DATA key);
> diff --git a/lib/ext2fs/undo_io.c b/lib/ext2fs/undo_io.c
> index d6beb02..94317cb 100644
> --- a/lib/ext2fs/undo_io.c
> +++ b/lib/ext2fs/undo_io.c
> @@ -37,6 +37,7 @@
> #if HAVE_SYS_RESOURCE_H
> #include <sys/resource.h>
> #endif
> +#include <limits.h>
>
> #include "tdb.h"
>
> @@ -354,8 +355,12 @@ static errcode_t undo_open(const char *name, int flags, io_channel *channel)
> data->real = 0;
> }
>
> + if (data->real)
> + io->flags = (io->flags & ~CHANNEL_FLAGS_DISCARD_ZEROES) |
> + (data->real->flags & CHANNEL_FLAGS_DISCARD_ZEROES);
> +
> /* setup the tdb file */
> - data->tdb = tdb_open(tdb_file, 0, TDB_CLEAR_IF_FIRST,
> + data->tdb = tdb_open(tdb_file, 0, TDB_CLEAR_IF_FIRST | TDB_NOLOCK | TDB_NOSYNC,
> O_RDWR | O_CREAT | O_TRUNC | O_EXCL, 0600);
> if (!data->tdb) {
> retval = errno;
> @@ -399,8 +404,10 @@ static errcode_t undo_close(io_channel channel)
> return retval;
> if (data->real)
> retval = io_channel_close(data->real);
> - if (data->tdb)
> + if (data->tdb) {
> + tdb_flush(data->tdb);
> tdb_close(data->tdb);
> + }
> ext2fs_free_mem(&channel->private_data);
> if (channel->name)
> ext2fs_free_mem(&channel->name);
> @@ -510,6 +517,77 @@ static errcode_t undo_write_byte(io_channel channel, unsigned long offset,
> return retval;
> }
>
> +static errcode_t undo_discard(io_channel channel, unsigned long long block,
> + unsigned long long count)
> +{
> + struct undo_private_data *data;
> + errcode_t retval = 0;
> + int icount;
> +
> + EXT2_CHECK_MAGIC(channel, EXT2_ET_MAGIC_IO_CHANNEL);
> + data = (struct undo_private_data *) channel->private_data;
> + EXT2_CHECK_MAGIC(data, EXT2_ET_MAGIC_UNIX_IO_CHANNEL);
> +
> + if (count > INT_MAX)
> + return EXT2_ET_UNIMPLEMENTED;
> + icount = count;
> +
> + /*
> + * First write the existing content into database
> + */
> + retval = undo_write_tdb(channel, block, icount);
> + if (retval)
> + return retval;
> + if (data->real)
> + retval = io_channel_discard(data->real, block, count);
> +
> + return retval;
> +}
> +
> +static errcode_t undo_zeroout(io_channel channel, unsigned long long block,
> + unsigned long long count)
> +{
> + struct undo_private_data *data;
> + errcode_t retval = 0;
> + int icount;
> +
> + EXT2_CHECK_MAGIC(channel, EXT2_ET_MAGIC_IO_CHANNEL);
> + data = (struct undo_private_data *) channel->private_data;
> + EXT2_CHECK_MAGIC(data, EXT2_ET_MAGIC_UNIX_IO_CHANNEL);
> +
> + if (count > INT_MAX)
> + return EXT2_ET_UNIMPLEMENTED;
> + icount = count;
> +
> + /*
> + * First write the existing content into database
> + */
> + retval = undo_write_tdb(channel, block, icount);
> + if (retval)
> + return retval;
> + if (data->real)
> + retval = io_channel_zeroout(data->real, block, count);
> +
> + return retval;
> +}
> +
> +static errcode_t undo_cache_readahead(io_channel channel,
> + unsigned long long block,
> + unsigned long long count)
> +{
> + struct undo_private_data *data;
> + errcode_t retval = 0;
> +
> + EXT2_CHECK_MAGIC(channel, EXT2_ET_MAGIC_IO_CHANNEL);
> + data = (struct undo_private_data *) channel->private_data;
> + EXT2_CHECK_MAGIC(data, EXT2_ET_MAGIC_UNIX_IO_CHANNEL);
> +
> + if (data->real)
> + retval = io_channel_cache_readahead(data->real, block, count);
> +
> + return retval;
> +}
> +
> /*
> * Flush data buffers to disk.
> */
> @@ -522,6 +600,8 @@ static errcode_t undo_flush(io_channel channel)
> data = (struct undo_private_data *) channel->private_data;
> EXT2_CHECK_MAGIC(data, EXT2_ET_MAGIC_UNIX_IO_CHANNEL);
>
> + if (data->tdb)
> + tdb_flush(data->tdb);
> if (data->real)
> retval = io_channel_flush(data->real);
>
> @@ -601,6 +681,9 @@ static struct struct_io_manager struct_undo_manager = {
> .get_stats = undo_get_stats,
> .read_blk64 = undo_read_blk64,
> .write_blk64 = undo_write_blk64,
> + .discard = undo_discard,
> + .zeroout = undo_zeroout,
> + .cache_readahead = undo_cache_readahead,
> };
>
> io_manager undo_io_manager = &struct_undo_manager;
>
> --
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