Commit 468dfca3 authored by Mikulas Patocka's avatar Mikulas Patocka Committed by Mike Snitzer
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dm integrity: add a bitmap mode 



Introduce an alternate mode of operation where dm-integrity uses a
bitmap instead of a journal. If a bit in the bitmap is 1, the
corresponding region's data and integrity tags are not synchronized - if
the machine crashes, the unsynchronized regions will be recalculated.
The bitmap mode is faster than the journal mode, because we don't have
to write the data twice, but it is also less reliable, because if data
corruption happens when the machine crashes, it may not be detected.

Benchmark results for an SSD connected to a SATA300 port, when doing
large linear writes with dd:

buffered I/O:
        raw device throughput - 245MB/s
        dm-integrity with journaling - 120MB/s
        dm-integrity with bitmap - 238MB/s

direct I/O with 1MB block size:
        raw device throughput - 248MB/s
        dm-integrity with journaling - 123MB/s
        dm-integrity with bitmap - 223MB/s

For more info see dm-integrity in Documentation/device-mapper/

Signed-off-by: default avatarMikulas Patocka <mpatocka@redhat.com>
Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
parent 8b3bbd49

Documentation/device-mapper/dm-integrity.txt

+22 −0
Original line number Diff line number Diff line
@@ -21,6 +21,13 @@ mode it calculates and verifies the integrity tag internally. In this 
mode, the dm-integrity target can be used to detect silent data

corruption on the disk or in the I/O path.



There's an alternate mode of operation where dm-integrity uses bitmap

instead of a journal. If a bit in the bitmap is 1, the corresponding

region's data and integrity tags are not synchronized - if the machine

crashes, the unsynchronized regions will be recalculated. The bitmap mode

is faster than the journal mode, because we don't have to write the data

twice, but it is also less reliable, because if data corruption happens

when the machine crashes, it may not be detected.



When loading the target for the first time, the kernel driver will format

the device. But it will only format the device if the superblock contains
@@ -59,6 +66,10 @@ Target arguments: 
		either both data and tag or none of them are written. The

		journaled mode degrades write throughput twice because the

		data have to be written twice.

	B - bitmap mode - data and metadata are written without any

		synchronization, the driver maintains a bitmap of dirty

		regions where data and metadata don't match. This mode can

		only be used with internal hash.

	R - recovery mode - in this mode, journal is not replayed,

		checksums are not checked and writes to the device are not

		allowed. This mode is useful for data recovery if the
@@ -150,6 +161,15 @@ block_size:number 
	Supported values are 512, 1024, 2048 and 4096 bytes.  If not

	specified the default block size is 512 bytes.



sectors_per_bit:number

	In the bitmap mode, this parameter specifies the number of

	512-byte sectors that corresponds to one bitmap bit.



bitmap_flush_interval:number

	The bitmap flush interval in milliseconds. The metadata buffers

	are synchronized when this interval expires.





The journal mode (D/J), buffer_sectors, journal_watermark, commit_time can

be changed when reloading the target (load an inactive table and swap the

tables with suspend and resume). The other arguments should not be changed
@@ -174,6 +194,8 @@ The layout of the formatted block device: 
	* flags

	  SB_FLAG_HAVE_JOURNAL_MAC - a flag is set if journal_mac is used

	  SB_FLAG_RECALCULATING - recalculating is in progress

	  SB_FLAG_DIRTY_BITMAP - journal area contains the bitmap of dirty

		blocks

	* log2(sectors per block)

	* a position where recalculating finished

* journal

drivers/md/dm-integrity.c

+503 −33

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