mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-11-25 05:04:09 +08:00
block: Data integrity infrastructure documentation
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com> Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
This commit is contained in:
Martin K. Petersen
Jens Axboe
parent
7ba1ba12ee
commit
c1c72b5994
@ -26,3 +26,37 @@ Description:
|
||||
I/O statistics of partition <part>. The format is the
|
||||
same as the above-written /sys/block/<disk>/stat
|
||||
format.
|
||||
|
||||
|
||||
What: /sys/block/<disk>/integrity/format
|
||||
Date: June 2008
|
||||
Contact: Martin K. Petersen <martin.petersen@oracle.com>
|
||||
Description:
|
||||
Metadata format for integrity capable block device.
|
||||
E.g. T10-DIF-TYPE1-CRC.
|
||||
|
||||
|
||||
What: /sys/block/<disk>/integrity/read_verify
|
||||
Date: June 2008
|
||||
Contact: Martin K. Petersen <martin.petersen@oracle.com>
|
||||
Description:
|
||||
Indicates whether the block layer should verify the
|
||||
integrity of read requests serviced by devices that
|
||||
support sending integrity metadata.
|
||||
|
||||
|
||||
What: /sys/block/<disk>/integrity/tag_size
|
||||
Date: June 2008
|
||||
Contact: Martin K. Petersen <martin.petersen@oracle.com>
|
||||
Description:
|
||||
Number of bytes of integrity tag space available per
|
||||
512 bytes of data.
|
||||
|
||||
|
||||
What: /sys/block/<disk>/integrity/write_generate
|
||||
Date: June 2008
|
||||
Contact: Martin K. Petersen <martin.petersen@oracle.com>
|
||||
Description:
|
||||
Indicates whether the block layer should automatically
|
||||
generate checksums for write requests bound for
|
||||
devices that support receiving integrity metadata.
|
||||
|
||||
327
Documentation/block/data-integrity.txt
Normal file
327
Documentation/block/data-integrity.txt
Normal file
@ -0,0 +1,327 @@
|
||||
----------------------------------------------------------------------
|
||||
1. INTRODUCTION
|
||||
|
||||
Modern filesystems feature checksumming of data and metadata to
|
||||
protect against data corruption. However, the detection of the
|
||||
corruption is done at read time which could potentially be months
|
||||
after the data was written. At that point the original data that the
|
||||
application tried to write is most likely lost.
|
||||
|
||||
The solution is to ensure that the disk is actually storing what the
|
||||
application meant it to. Recent additions to both the SCSI family
|
||||
protocols (SBC Data Integrity Field, SCC protection proposal) as well
|
||||
as SATA/T13 (External Path Protection) try to remedy this by adding
|
||||
support for appending integrity metadata to an I/O. The integrity
|
||||
metadata (or protection information in SCSI terminology) includes a
|
||||
checksum for each sector as well as an incrementing counter that
|
||||
ensures the individual sectors are written in the right order. And
|
||||
for some protection schemes also that the I/O is written to the right
|
||||
place on disk.
|
||||
|
||||
Current storage controllers and devices implement various protective
|
||||
measures, for instance checksumming and scrubbing. But these
|
||||
technologies are working in their own isolated domains or at best
|
||||
between adjacent nodes in the I/O path. The interesting thing about
|
||||
DIF and the other integrity extensions is that the protection format
|
||||
is well defined and every node in the I/O path can verify the
|
||||
integrity of the I/O and reject it if corruption is detected. This
|
||||
allows not only corruption prevention but also isolation of the point
|
||||
of failure.
|
||||
|
||||
----------------------------------------------------------------------
|
||||
2. THE DATA INTEGRITY EXTENSIONS
|
||||
|
||||
As written, the protocol extensions only protect the path between
|
||||
controller and storage device. However, many controllers actually
|
||||
allow the operating system to interact with the integrity metadata
|
||||
(IMD). We have been working with several FC/SAS HBA vendors to enable
|
||||
the protection information to be transferred to and from their
|
||||
controllers.
|
||||
|
||||
The SCSI Data Integrity Field works by appending 8 bytes of protection
|
||||
information to each sector. The data + integrity metadata is stored
|
||||
in 520 byte sectors on disk. Data + IMD are interleaved when
|
||||
transferred between the controller and target. The T13 proposal is
|
||||
similar.
|
||||
|
||||
Because it is highly inconvenient for operating systems to deal with
|
||||
520 (and 4104) byte sectors, we approached several HBA vendors and
|
||||
encouraged them to allow separation of the data and integrity metadata
|
||||
scatter-gather lists.
|
||||
|
||||
The controller will interleave the buffers on write and split them on
|
||||
read. This means that the Linux can DMA the data buffers to and from
|
||||
host memory without changes to the page cache.
|
||||
|
||||
Also, the 16-bit CRC checksum mandated by both the SCSI and SATA specs
|
||||
is somewhat heavy to compute in software. Benchmarks found that
|
||||
calculating this checksum had a significant impact on system
|
||||
performance for a number of workloads. Some controllers allow a
|
||||
lighter-weight checksum to be used when interfacing with the operating
|
||||
system. Emulex, for instance, supports the TCP/IP checksum instead.
|
||||
The IP checksum received from the OS is converted to the 16-bit CRC
|
||||
when writing and vice versa. This allows the integrity metadata to be
|
||||
generated by Linux or the application at very low cost (comparable to
|
||||
software RAID5).
|
||||
|
||||
The IP checksum is weaker than the CRC in terms of detecting bit
|
||||
errors. However, the strength is really in the separation of the data
|
||||
buffers and the integrity metadata. These two distinct buffers much
|
||||
match up for an I/O to complete.
|
||||
|
||||
The separation of the data and integrity metadata buffers as well as
|
||||
the choice in checksums is referred to as the Data Integrity
|
||||
Extensions. As these extensions are outside the scope of the protocol
|
||||
bodies (T10, T13), Oracle and its partners are trying to standardize
|
||||
them within the Storage Networking Industry Association.
|
||||
|
||||
----------------------------------------------------------------------
|
||||
3. KERNEL CHANGES
|
||||
|
||||
The data integrity framework in Linux enables protection information
|
||||
to be pinned to I/Os and sent to/received from controllers that
|
||||
support it.
|
||||
|
||||
The advantage to the integrity extensions in SCSI and SATA is that
|
||||
they enable us to protect the entire path from application to storage
|
||||
device. However, at the same time this is also the biggest
|
||||
disadvantage. It means that the protection information must be in a
|
||||
format that can be understood by the disk.
|
||||
|
||||
Generally Linux/POSIX applications are agnostic to the intricacies of
|
||||
the storage devices they are accessing. The virtual filesystem switch
|
||||
and the block layer make things like hardware sector size and
|
||||
transport protocols completely transparent to the application.
|
||||
|
||||
However, this level of detail is required when preparing the
|
||||
protection information to send to a disk. Consequently, the very
|
||||
concept of an end-to-end protection scheme is a layering violation.
|
||||
It is completely unreasonable for an application to be aware whether
|
||||
it is accessing a SCSI or SATA disk.
|
||||
|
||||
The data integrity support implemented in Linux attempts to hide this
|
||||
from the application. As far as the application (and to some extent
|
||||
the kernel) is concerned, the integrity metadata is opaque information
|
||||
that's attached to the I/O.
|
||||
|
||||
The current implementation allows the block layer to automatically
|
||||
generate the protection information for any I/O. Eventually the
|
||||
intent is to move the integrity metadata calculation to userspace for
|
||||
user data. Metadata and other I/O that originates within the kernel
|
||||
will still use the automatic generation interface.
|
||||
|
||||
Some storage devices allow each hardware sector to be tagged with a
|
||||
16-bit value. The owner of this tag space is the owner of the block
|
||||
device. I.e. the filesystem in most cases. The filesystem can use
|
||||
this extra space to tag sectors as they see fit. Because the tag
|
||||
space is limited, the block interface allows tagging bigger chunks by
|
||||
way of interleaving. This way, 8*16 bits of information can be
|
||||
attached to a typical 4KB filesystem block.
|
||||
|
||||
This also means that applications such as fsck and mkfs will need
|
||||
access to manipulate the tags from user space. A passthrough
|
||||
interface for this is being worked on.
|
||||
|
||||
|
||||
----------------------------------------------------------------------
|
||||
4. BLOCK LAYER IMPLEMENTATION DETAILS
|
||||
|
||||
4.1 BIO
|
||||
|
||||
The data integrity patches add a new field to struct bio when
|
||||
CONFIG_BLK_DEV_INTEGRITY is enabled. bio->bi_integrity is a pointer
|
||||
to a struct bip which contains the bio integrity payload. Essentially
|
||||
a bip is a trimmed down struct bio which holds a bio_vec containing
|
||||
the integrity metadata and the required housekeeping information (bvec
|
||||
pool, vector count, etc.)
|
||||
|
||||
A kernel subsystem can enable data integrity protection on a bio by
|
||||
calling bio_integrity_alloc(bio). This will allocate and attach the
|
||||
bip to the bio.
|
||||
|
||||
Individual pages containing integrity metadata can subsequently be
|
||||
attached using bio_integrity_add_page().
|
||||
|
||||
bio_free() will automatically free the bip.
|
||||
|
||||
|
||||
4.2 BLOCK DEVICE
|
||||
|
||||
Because the format of the protection data is tied to the physical
|
||||
disk, each block device has been extended with a block integrity
|
||||
profile (struct blk_integrity). This optional profile is registered
|
||||
with the block layer using blk_integrity_register().
|
||||
|
||||
The profile contains callback functions for generating and verifying
|
||||
the protection data, as well as getting and setting application tags.
|
||||
The profile also contains a few constants to aid in completing,
|
||||
merging and splitting the integrity metadata.
|
||||
|
||||
Layered block devices will need to pick a profile that's appropriate
|
||||
for all subdevices. blk_integrity_compare() can help with that. DM
|
||||
and MD linear, RAID0 and RAID1 are currently supported. RAID4/5/6
|
||||
will require extra work due to the application tag.
|
||||
|
||||
|
||||
----------------------------------------------------------------------
|
||||
5.0 BLOCK LAYER INTEGRITY API
|
||||
|
||||
5.1 NORMAL FILESYSTEM
|
||||
|
||||
The normal filesystem is unaware that the underlying block device
|
||||
is capable of sending/receiving integrity metadata. The IMD will
|
||||
be automatically generated by the block layer at submit_bio() time
|
||||
in case of a WRITE. A READ request will cause the I/O integrity
|
||||
to be verified upon completion.
|
||||
|
||||
IMD generation and verification can be toggled using the
|
||||
|
||||
/sys/block/<bdev>/integrity/write_generate
|
||||
|
||||
and
|
||||
|
||||
/sys/block/<bdev>/integrity/read_verify
|
||||
|
||||
flags.
|
||||
|
||||
|
||||
5.2 INTEGRITY-AWARE FILESYSTEM
|
||||
|
||||
A filesystem that is integrity-aware can prepare I/Os with IMD
|
||||
attached. It can also use the application tag space if this is
|
||||
supported by the block device.
|
||||
|
||||
|
||||
int bdev_integrity_enabled(block_device, int rw);
|
||||
|
||||
bdev_integrity_enabled() will return 1 if the block device
|
||||
supports integrity metadata transfer for the data direction
|
||||
specified in 'rw'.
|
||||
|
||||
bdev_integrity_enabled() honors the write_generate and
|
||||
read_verify flags in sysfs and will respond accordingly.
|
||||
|
||||
|
||||
int bio_integrity_prep(bio);
|
||||
|
||||
To generate IMD for WRITE and to set up buffers for READ, the
|
||||
filesystem must call bio_integrity_prep(bio).
|
||||
|
||||
Prior to calling this function, the bio data direction and start
|
||||
sector must be set, and the bio should have all data pages
|
||||
added. It is up to the caller to ensure that the bio does not
|
||||
change while I/O is in progress.
|
||||
|
||||
bio_integrity_prep() should only be called if
|
||||
bio_integrity_enabled() returned 1.
|
||||
|
||||
|
||||
int bio_integrity_tag_size(bio);
|
||||
|
||||
If the filesystem wants to use the application tag space it will
|
||||
first have to find out how much storage space is available.
|
||||
Because tag space is generally limited (usually 2 bytes per
|
||||
sector regardless of sector size), the integrity framework
|
||||
supports interleaving the information between the sectors in an
|
||||
I/O.
|
||||
|
||||
Filesystems can call bio_integrity_tag_size(bio) to find out how
|
||||
many bytes of storage are available for that particular bio.
|
||||
|
||||
Another option is bdev_get_tag_size(block_device) which will
|
||||
return the number of available bytes per hardware sector.
|
||||
|
||||
|
||||
int bio_integrity_set_tag(bio, void *tag_buf, len);
|
||||
|
||||
After a successful return from bio_integrity_prep(),
|
||||
bio_integrity_set_tag() can be used to attach an opaque tag
|
||||
buffer to a bio. Obviously this only makes sense if the I/O is
|
||||
a WRITE.
|
||||
|
||||
|
||||
int bio_integrity_get_tag(bio, void *tag_buf, len);
|
||||
|
||||
Similarly, at READ I/O completion time the filesystem can
|
||||
retrieve the tag buffer using bio_integrity_get_tag().
|
||||
|
||||
|
||||
6.3 PASSING EXISTING INTEGRITY METADATA
|
||||
|
||||
Filesystems that either generate their own integrity metadata or
|
||||
are capable of transferring IMD from user space can use the
|
||||
following calls:
|
||||
|
||||
|
||||
struct bip * bio_integrity_alloc(bio, gfp_mask, nr_pages);
|
||||
|
||||
Allocates the bio integrity payload and hangs it off of the bio.
|
||||
nr_pages indicate how many pages of protection data need to be
|
||||
stored in the integrity bio_vec list (similar to bio_alloc()).
|
||||
|
||||
The integrity payload will be freed at bio_free() time.
|
||||
|
||||
|
||||
int bio_integrity_add_page(bio, page, len, offset);
|
||||
|
||||
Attaches a page containing integrity metadata to an existing
|
||||
bio. The bio must have an existing bip,
|
||||
i.e. bio_integrity_alloc() must have been called. For a WRITE,
|
||||
the integrity metadata in the pages must be in a format
|
||||
understood by the target device with the notable exception that
|
||||
the sector numbers will be remapped as the request traverses the
|
||||
I/O stack. This implies that the pages added using this call
|
||||
will be modified during I/O! The first reference tag in the
|
||||
integrity metadata must have a value of bip->bip_sector.
|
||||
|
||||
Pages can be added using bio_integrity_add_page() as long as
|
||||
there is room in the bip bio_vec array (nr_pages).
|
||||
|
||||
Upon completion of a READ operation, the attached pages will
|
||||
contain the integrity metadata received from the storage device.
|
||||
It is up to the receiver to process them and verify data
|
||||
integrity upon completion.
|
||||
|
||||
|
||||
6.4 REGISTERING A BLOCK DEVICE AS CAPABLE OF EXCHANGING INTEGRITY
|
||||
METADATA
|
||||
|
||||
To enable integrity exchange on a block device the gendisk must be
|
||||
registered as capable:
|
||||
|
||||
int blk_integrity_register(gendisk, blk_integrity);
|
||||
|
||||
The blk_integrity struct is a template and should contain the
|
||||
following:
|
||||
|
||||
static struct blk_integrity my_profile = {
|
||||
.name = "STANDARDSBODY-TYPE-VARIANT-CSUM",
|
||||
.generate_fn = my_generate_fn,
|
||||
.verify_fn = my_verify_fn,
|
||||
.get_tag_fn = my_get_tag_fn,
|
||||
.set_tag_fn = my_set_tag_fn,
|
||||
.tuple_size = sizeof(struct my_tuple_size),
|
||||
.tag_size = <tag bytes per hw sector>,
|
||||
};
|
||||
|
||||
'name' is a text string which will be visible in sysfs. This is
|
||||
part of the userland API so chose it carefully and never change
|
||||
it. The format is standards body-type-variant.
|
||||
E.g. T10-DIF-TYPE1-IP or T13-EPP-0-CRC.
|
||||
|
||||
'generate_fn' generates appropriate integrity metadata (for WRITE).
|
||||
|
||||
'verify_fn' verifies that the data buffer matches the integrity
|
||||
metadata.
|
||||
|
||||
'tuple_size' must be set to match the size of the integrity
|
||||
metadata per sector. I.e. 8 for DIF and EPP.
|
||||
|
||||
'tag_size' must be set to identify how many bytes of tag space
|
||||
are available per hardware sector. For DIF this is either 2 or
|
||||
0 depending on the value of the Control Mode Page ATO bit.
|
||||
|
||||
See 6.2 for a description of get_tag_fn and set_tag_fn.
|
||||
|
||||
----------------------------------------------------------------------
|
||||
2007-12-24 Martin K. Petersen <martin.petersen@oracle.com>
|
||||
Loading…
Reference in New Issue
Block a user