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diff --git a/doc/dev/file-striping.rst b/doc/dev/file-striping.rst new file mode 100644 index 000000000..405c9718d --- /dev/null +++ b/doc/dev/file-striping.rst @@ -0,0 +1,161 @@ +File striping +============= + +The text below describes how files from Ceph file system clients are +stored across objects stored in RADOS. + +ceph_file_layout +---------------- + +Ceph distributes (stripes) the data for a given file across a number +of underlying objects. The way file data is mapped to those objects +is defined by the ceph_file_layout structure. The data distribution +is a modified RAID 0, where data is striped across a set of objects up +to a (per-file) fixed size, at which point another set of objects +holds the file's data. The second set also holds no more than the +fixed amount of data, and then another set is used, and so on. + +Defining some terminology will go a long way toward explaining the +way file data is laid out across Ceph objects. + +- file + A collection of contiguous data, named from the perspective of + the Ceph client (i.e., a file on a Linux system using Ceph + storage). The data for a file is divided into fixed-size + "stripe units," which are stored in ceph "objects." +- stripe unit + The size (in bytes) of a block of data used in the RAID 0 + distribution of a file. All stripe units for a file have equal + size. The last stripe unit is typically incomplete--i.e. it + represents the data at the end of the file as well as unused + "space" beyond it up to the end of the fixed stripe unit size. +- stripe count + The number of consecutive stripe units that constitute a RAID 0 + "stripe" of file data. +- stripe + A contiguous range of file data, RAID 0 striped across "stripe + count" objects in fixed-size "stripe unit" blocks. +- object + A collection of data maintained by Ceph storage. Objects are + used to hold portions of Ceph client files. +- object set + A set of objects that together represent a contiguous portion of + a file. + +Three fields in the ceph_file_layout structure define this mapping:: + + u32 fl_stripe_unit; + u32 fl_stripe_count; + u32 fl_object_size; + +(They are actually maintained in their on-disk format, __le32.) + +The role of the first two fields should be clear from the +definitions above. + +The third field is the maximum size (in bytes) of an object used to +back file data. The object size is a multiple of the stripe unit. + +A file's data is blocked into stripe units, and consecutive stripe +units are stored on objects in an object set. The number of objects +in a set is the same as the stripe count. No object storing file +data will exceed the file's designated object size, so after some +fixed number of complete stripes, a new object set is used to store +subsequent file data. + +Note that by default, Ceph uses a simple striping strategy in which +object_size equals stripe_unit and stripe_count is 1. This simply +puts one stripe_unit in each object. + +Here's a more complex example:: + + file size = 1 trillion = 1000000000000 bytes + + fl_stripe_unit = 64KB = 65536 bytes + fl_stripe_count = 5 stripe units per stripe + fl_object_size = 64GB = 68719476736 bytes + +This means:: + + file stripe size = 64KB * 5 = 320KB = 327680 bytes + each object holds 64GB / 64KB = 1048576 stripe units + file object set size = 64GB * 5 = 320GB = 343597383680 bytes + (also 1048576 stripe units * 327680 bytes per stripe unit) + +So the file's 1 trillion bytes can be divided into complete object +sets, then complete stripes, then complete stripe units, and finally +a single incomplete stripe unit:: + + - 1 trillion bytes / 320GB per object set = 2 complete object sets + (with 312805232640 bytes remaining) + - 312805232640 bytes / 320KB per stripe = 954605 complete stripes + (with 266240 bytes remaining) + - 266240 bytes / 64KB per stripe unit = 4 complete stripe units + (with 4096 bytes remaining) + - and the final incomplete stripe unit holds those 4096 bytes. + +The ASCII art below attempts to capture this:: + + _________ _________ _________ _________ _________ + /object 0\ /object 1\ /object 2\ /object 3\ /object 4\ + +=========+ +=========+ +=========+ +=========+ +=========+ + | stripe | | stripe | | stripe | | stripe | | stripe | + o | unit | | unit | | unit | | unit | | unit | stripe 0 + b | 0 | | 1 | | 2 | | 3 | | 4 | + j |---------| |---------| |---------| |---------| |---------| + e | stripe | | stripe | | stripe | | stripe | | stripe | + c | unit | | unit | | unit | | unit | | unit | stripe 1 + t | 5 | | 6 | | 7 | | 8 | | 9 | + |---------| |---------| |---------| |---------| |---------| + s | . | | . | | . | | . | | . | + e . . . . . + t | . | | . | | . | | . | | . | + |---------| |---------| |---------| |---------| |---------| + 0 | stripe | | stripe | | stripe | | stripe | | stripe | stripe + | unit | | unit | | unit | | unit | | unit | 1048575 + | 5242875 | | 5242876 | | 5242877 | | 5242878 | | 5242879 | + \=========/ \=========/ \=========/ \=========/ \=========/ + + _________ _________ _________ _________ _________ + /object 5\ /object 6\ /object 7\ /object 8\ /object 9\ + +=========+ +=========+ +=========+ +=========+ +=========+ + | stripe | | stripe | | stripe | | stripe | | stripe | stripe + o | unit | | unit | | unit | | unit | | unit | 1048576 + b | 5242880 | | 5242881 | | 5242882 | | 5242883 | | 5242884 | + j |---------| |---------| |---------| |---------| |---------| + e | stripe | | stripe | | stripe | | stripe | | stripe | stripe + c | unit | | unit | | unit | | unit | | unit | 1048577 + t | 5242885 | | 5242886 | | 5242887 | | 5242888 | | 5242889 | + |---------| |---------| |---------| |---------| |---------| + s | . | | . | | . | | . | | . | + e . . . . . + t | . | | . | | . | | . | | . | + |---------| |---------| |---------| |---------| |---------| + 1 | stripe | | stripe | | stripe | | stripe | | stripe | stripe + | unit | | unit | | unit | | unit | | unit | 2097151 + | 10485755| | 10485756| | 10485757| | 10485758| | 10485759| + \=========/ \=========/ \=========/ \=========/ \=========/ + + _________ _________ _________ _________ _________ + /object 10\ /object 11\ /object 12\ /object 13\ /object 14\ + +=========+ +=========+ +=========+ +=========+ +=========+ + | stripe | | stripe | | stripe | | stripe | | stripe | stripe + o | unit | | unit | | unit | | unit | | unit | 2097152 + b | 10485760| | 10485761| | 10485762| | 10485763| | 10485764| + j |---------| |---------| |---------| |---------| |---------| + e | stripe | | stripe | | stripe | | stripe | | stripe | stripe + c | unit | | unit | | unit | | unit | | unit | 2097153 + t | 10485765| | 10485766| | 10485767| | 10485768| | 10485769| + |---------| |---------| |---------| |---------| |---------| + s | . | | . | | . | | . | | . | + e . . . . . + t | . | | . | | . | | . | | . | + |---------| |---------| |---------| |---------| |---------| + 2 | stripe | | stripe | | stripe | | stripe | | stripe | stripe + | unit | | unit | | unit | | unit | | unit | 3051756 + | 15258780| | 15258781| | 15258782| | 15258783| | 15258784| + |---------| |---------| |---------| |---------| |---------| + | stripe | | stripe | | stripe | | stripe | | (partial| (partial + | unit | | unit | | unit | | unit | | stripe | stripe + | 15258785| | 15258786| | 15258787| | 15258788| | unit) | 3051757) + \=========/ \=========/ \=========/ \=========/ \=========/ |