The kernel default is too low, 32 MiB is recommended and should give
better results.
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
---
Documentation/btrfs-filesystem.asciidoc | 16 ++++++++++++----
cmds-filesystem.c | 11 +++++++++--
2 files changed, 21 insertions(+), 6 deletions(-)
diff --git a/Documentation/btrfs-filesystem.asciidoc b/Documentation/btrfs-filesystem.asciidoc
index 8fb15eacafc9..28e772466cfa 100644
--- a/Documentation/btrfs-filesystem.asciidoc
+++ b/Documentation/btrfs-filesystem.asciidoc
@@ -117,15 +117,23 @@ compression. See also section 'EXAMPLES'.
-r::::
defragment files recursively in given directories
-f::::
-flush data for each file before going to the next file. This will limit the amount
-of dirty data to current file, otherwise the amount cumulates from several files
-and may increase system load.
+flush data for each file before going to the next file.
++
+This will limit the amount of dirty data to current file, otherwise the amount
+cumulates from several files and will increase system load. This can also lead
+to ENOSPC if there's too much dirty data to write and it's not possible to make
+the reservations for the new data (ie. how the COW design works).
++
-s <start>[kKmMgGtTpPeE]::::
defragmentation will start from the given offset, default is beginning of a file
-l <len>[kKmMgGtTpPeE]::::
defragment only up to 'len' bytes, default is the file size
-t <size>[kKmMgGtTpPeE]::::
-target extent size, do not touch extents bigger than 'size'
+target extent size, do not touch extents bigger than 'size', default: 32 MiB
++
+The value is only advisory and the final size of the extents may differ,
+depending on the state of the free space and fragmentation or other internal
+logic. Reasonable values are from tens to hundreds of megabytes.
*du* [options] <path> [<path>..]::
Calculate disk usage of the target files using FIEMAP. For individual
diff --git a/cmds-filesystem.c b/cmds-filesystem.c
index ef1f550b51c0..6b381c582ea7 100644
--- a/cmds-filesystem.c
+++ b/cmds-filesystem.c
@@ -968,7 +968,7 @@ static const char * const cmd_filesystem_defrag_usage[] = {
"-f flush data to disk immediately after defragmenting",
"-s start defragment only from byte onward",
"-l len defragment only up to len bytes",
- "-t size target extent size hint",
+ "-t size target extent size hint (default: 32 MiB)",
NULL
};
@@ -1029,7 +1029,7 @@ static int cmd_filesystem_defrag(int argc, char **argv)
int flush = 0;
u64 start = 0;
u64 len = (u64)-1;
- u64 thresh = 0;
+ u64 thresh;
int i;
int recursive = 0;
int ret = 0;
@@ -1037,6 +1037,13 @@ static int cmd_filesystem_defrag(int argc, char **argv)
int compress_type = BTRFS_COMPRESS_NONE;
DIR *dirstream;
+ /*
+ * Kernel has a different default (256K) that is supposed to be safe,
+ * but it does not defragment very well. The 32M will likely lead to
+ * better results and is independent of the kernel default.
+ */
+ thresh = 32 * 1024 * 1024;
+
defrag_global_errors = 0;
defrag_global_verbose = 0;
defrag_global_errors = 0;
--
2.7.1
--
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