mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-11-19 02:04:19 +08:00
898bd37a92
Rename the block documentation files to ReST, add an index for them and adjust in order to produce a nice html output via the Sphinx build system. At its new index.rst, let's add a :orphan: while this is not linked to the main index.rst file, in order to avoid build warnings. Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
292 lines
12 KiB
ReStructuredText
292 lines
12 KiB
ReStructuredText
==============
|
|
Data Integrity
|
|
==============
|
|
|
|
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 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 must
|
|
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_integrity(bio) returns 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.
|
|
|
|
|
|
`bool 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.
|
|
Complete bio with error if prepare failed for some reson.
|
|
|
|
|
|
5.3 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.
|
|
|
|
|
|
5.4 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,
|
|
.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.
|
|
|
|
----------------------------------------------------------------------
|
|
|
|
2007-12-24 Martin K. Petersen <martin.petersen@oracle.com>
|