Btrfs send reads data from disk and then writes to a stream via pipe or
a file via flush.
Currently we're going to read each page a time, so every page results
in a disk read, which is not friendly to disks, esp. HDD. Given that,
the performance can be gained by adding readahead for those pages.
Here is a quick test:
$ btrfs subvolume create send
$ xfs_io -f -c "pwrite 0 1G" send/foobar
$ btrfs subvolume snap -r send ro
$ time "btrfs send ro -f /dev/null"
w/o w
real 1m37.527s 0m9.097s
user 0m0.122s 0m0.086s
sys 0m53.191s 0m12.857s
Signed-off-by: Liu Bo <bo.li.liu@xxxxxxxxxx>
---
v2->v3: move file_ra_state to @send_ctx, suggested by David Sterba.
v1->v2: return ENOMEM on failing to allocate memory.
fs/btrfs/send.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
index 9dde971..190a71e 100644
--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -120,6 +120,8 @@ struct send_ctx {
struct list_head name_cache_list;
int name_cache_size;
+ struct file_ra_state ra;
+
char *read_buf;
/*
@@ -3991,6 +3993,13 @@ static ssize_t fill_read_buf(struct send_ctx *sctx, u64 offset, u32 len)
goto out;
last_index = (offset + len - 1) >> PAGE_CACHE_SHIFT;
+
+ /* initial readahead */
+ memset(&sctx->ra, 0, sizeof(struct file_ra_state));
+ file_ra_state_init(&sctx->ra, inode->i_mapping);
+ btrfs_force_ra(inode->i_mapping, &sctx->ra, NULL, index,
+ last_index - index + 1);
+
while (index <= last_index) {
unsigned cur_len = min_t(unsigned, len,
PAGE_CACHE_SIZE - pg_offset);
--
1.8.1.4
--
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