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162 lines
7.7 KiB
ReStructuredText
162 lines
7.7 KiB
ReStructuredText
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File striping
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=============
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The text below describes how files from Ceph file system clients are
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stored across objects stored in RADOS.
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ceph_file_layout
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----------------
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Ceph distributes (stripes) the data for a given file across a number
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of underlying objects. The way file data is mapped to those objects
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is defined by the ceph_file_layout structure. The data distribution
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is a modified RAID 0, where data is striped across a set of objects up
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to a (per-file) fixed size, at which point another set of objects
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holds the file's data. The second set also holds no more than the
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fixed amount of data, and then another set is used, and so on.
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Defining some terminology will go a long way toward explaining the
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way file data is laid out across Ceph objects.
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- file
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A collection of contiguous data, named from the perspective of
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the Ceph client (i.e., a file on a Linux system using Ceph
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storage). The data for a file is divided into fixed-size
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"stripe units," which are stored in ceph "objects."
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- stripe unit
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The size (in bytes) of a block of data used in the RAID 0
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distribution of a file. All stripe units for a file have equal
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size. The last stripe unit is typically incomplete--i.e. it
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represents the data at the end of the file as well as unused
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"space" beyond it up to the end of the fixed stripe unit size.
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- stripe count
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The number of consecutive stripe units that constitute a RAID 0
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"stripe" of file data.
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- stripe
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A contiguous range of file data, RAID 0 striped across "stripe
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count" objects in fixed-size "stripe unit" blocks.
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- object
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A collection of data maintained by Ceph storage. Objects are
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used to hold portions of Ceph client files.
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- object set
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A set of objects that together represent a contiguous portion of
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a file.
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Three fields in the ceph_file_layout structure define this mapping::
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u32 fl_stripe_unit;
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u32 fl_stripe_count;
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u32 fl_object_size;
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(They are actually maintained in their on-disk format, __le32.)
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The role of the first two fields should be clear from the
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definitions above.
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The third field is the maximum size (in bytes) of an object used to
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back file data. The object size is a multiple of the stripe unit.
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A file's data is blocked into stripe units, and consecutive stripe
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units are stored on objects in an object set. The number of objects
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in a set is the same as the stripe count. No object storing file
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data will exceed the file's designated object size, so after some
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fixed number of complete stripes, a new object set is used to store
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subsequent file data.
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Note that by default, Ceph uses a simple striping strategy in which
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object_size equals stripe_unit and stripe_count is 1. This simply
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puts one stripe_unit in each object.
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Here's a more complex example::
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file size = 1 trillion = 1000000000000 bytes
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fl_stripe_unit = 64KB = 65536 bytes
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fl_stripe_count = 5 stripe units per stripe
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fl_object_size = 64GB = 68719476736 bytes
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This means::
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file stripe size = 64KB * 5 = 320KB = 327680 bytes
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each object holds 64GB / 64KB = 1048576 stripe units
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file object set size = 64GB * 5 = 320GB = 343597383680 bytes
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(also 1048576 stripe units * 327680 bytes per stripe unit)
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So the file's 1 trillion bytes can be divided into complete object
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sets, then complete stripes, then complete stripe units, and finally
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a single incomplete stripe unit::
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- 1 trillion bytes / 320GB per object set = 2 complete object sets
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(with 312805232640 bytes remaining)
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- 312805232640 bytes / 320KB per stripe = 954605 complete stripes
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(with 266240 bytes remaining)
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- 266240 bytes / 64KB per stripe unit = 4 complete stripe units
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(with 4096 bytes remaining)
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- and the final incomplete stripe unit holds those 4096 bytes.
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The ASCII art below attempts to capture this::
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_________ _________ _________ _________ _________
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/object 0\ /object 1\ /object 2\ /object 3\ /object 4\
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+=========+ +=========+ +=========+ +=========+ +=========+
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| stripe | | stripe | | stripe | | stripe | | stripe |
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o | unit | | unit | | unit | | unit | | unit | stripe 0
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b | 0 | | 1 | | 2 | | 3 | | 4 |
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j |---------| |---------| |---------| |---------| |---------|
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e | stripe | | stripe | | stripe | | stripe | | stripe |
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c | unit | | unit | | unit | | unit | | unit | stripe 1
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t | 5 | | 6 | | 7 | | 8 | | 9 |
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|---------| |---------| |---------| |---------| |---------|
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s | . | | . | | . | | . | | . |
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e . . . . .
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t | . | | . | | . | | . | | . |
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|---------| |---------| |---------| |---------| |---------|
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0 | stripe | | stripe | | stripe | | stripe | | stripe | stripe
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| unit | | unit | | unit | | unit | | unit | 1048575
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| 5242875 | | 5242876 | | 5242877 | | 5242878 | | 5242879 |
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\=========/ \=========/ \=========/ \=========/ \=========/
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_________ _________ _________ _________ _________
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/object 5\ /object 6\ /object 7\ /object 8\ /object 9\
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+=========+ +=========+ +=========+ +=========+ +=========+
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| stripe | | stripe | | stripe | | stripe | | stripe | stripe
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o | unit | | unit | | unit | | unit | | unit | 1048576
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b | 5242880 | | 5242881 | | 5242882 | | 5242883 | | 5242884 |
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j |---------| |---------| |---------| |---------| |---------|
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e | stripe | | stripe | | stripe | | stripe | | stripe | stripe
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c | unit | | unit | | unit | | unit | | unit | 1048577
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t | 5242885 | | 5242886 | | 5242887 | | 5242888 | | 5242889 |
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|---------| |---------| |---------| |---------| |---------|
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s | . | | . | | . | | . | | . |
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e . . . . .
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t | . | | . | | . | | . | | . |
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|---------| |---------| |---------| |---------| |---------|
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1 | stripe | | stripe | | stripe | | stripe | | stripe | stripe
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| unit | | unit | | unit | | unit | | unit | 2097151
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| 10485755| | 10485756| | 10485757| | 10485758| | 10485759|
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\=========/ \=========/ \=========/ \=========/ \=========/
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_________ _________ _________ _________ _________
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/object 10\ /object 11\ /object 12\ /object 13\ /object 14\
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+=========+ +=========+ +=========+ +=========+ +=========+
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| stripe | | stripe | | stripe | | stripe | | stripe | stripe
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o | unit | | unit | | unit | | unit | | unit | 2097152
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b | 10485760| | 10485761| | 10485762| | 10485763| | 10485764|
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j |---------| |---------| |---------| |---------| |---------|
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e | stripe | | stripe | | stripe | | stripe | | stripe | stripe
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c | unit | | unit | | unit | | unit | | unit | 2097153
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t | 10485765| | 10485766| | 10485767| | 10485768| | 10485769|
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|---------| |---------| |---------| |---------| |---------|
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s | . | | . | | . | | . | | . |
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e . . . . .
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t | . | | . | | . | | . | | . |
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|---------| |---------| |---------| |---------| |---------|
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2 | stripe | | stripe | | stripe | | stripe | | stripe | stripe
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| unit | | unit | | unit | | unit | | unit | 3051756
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| 15258780| | 15258781| | 15258782| | 15258783| | 15258784|
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|---------| |---------| |---------| |---------| |---------|
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| stripe | | stripe | | stripe | | stripe | | (partial| (partial
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| unit | | unit | | unit | | unit | | stripe | stripe
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| 15258785| | 15258786| | 15258787| | 15258788| | unit) | 3051757)
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\=========/ \=========/ \=========/ \=========/ \=========/
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