From: Liu Yuan <tailai.ly at taobao.com> Signed-off-by: Liu Yuan <tailai.ly at taobao.com> --- doc/farm-internal.txt | 95 +++++++++++++++++++++++++++++++++++++++++++++++++ 1 files changed, 95 insertions(+), 0 deletions(-) create mode 100644 doc/farm-internal.txt diff --git a/doc/farm-internal.txt b/doc/farm-internal.txt new file mode 100644 index 0000000..8e7587c --- /dev/null +++ b/doc/farm-internal.txt @@ -0,0 +1,95 @@ + ================== + Farm Store + ================== + + Liu Yuan <namei.unix at gmail.com> Taobao Inc. + +1. OVERVIEW + +Simply put, Farm somewhat resembles git a lot (both code and idea level). +there are three object type, named 'data, trunk, snapshot[*]' that is +similar to git's 'blob, tree, commit'. + +[*] shorten to 'snap' below. + +'data' object is just Sheepdog's I/O object, only named by its sha1-ed +content. So the data objects with the same content will be mapped to only +single sha1 file, thus achieve node-wide data sharing. + +'trunk' object ties data objects together into a flat directory structure at +the time of the snapshot being taken. The trunk object provides a means to +find old data objects in the store. + +'snap' object describes the snapshot, either initiated by users or triggered +by recovery code. The snap object refers to one of the trunk objects. The two +snap log files provides a means to name the desired snap object. + +All the objects are depicted in the context of snapshotting or retrieving old +data from the snapshotted objects, that is, those objects are 'cached' into +Farm store by performing snapshotting operations. + +Farm has a working directory that is the ground for storing objebts from Sheepdog +I/O requests. That being said, the I/O performance for VM Guests would be +practically the same as Simple Store. + +2. OBJECT LAYOUT + +All the objects(snap, trunk, data) in the Farm is based on the operations of +the sha1_file. sha1_file provides us compressed and consistency-aware +characteristics independent of content or the type of the object. + +The object successfully inflates to a stream of bytes that forms a sequence of + + <sha1_file_hdr> + <binary object data> + | | + header payload + +The payload of the data object is the compressed content of Sheepdog's I/O object. + +For trunk object, the compressed content is + + <array of the struct trunk_entry> + + struct trunk_entry { + uint64_t oid; + unsigned char sha1[SHA1_LEN]; + }; + +For snap object, the compressed content is + + <trunk_sha1> + <array of the struct sd_node> + +As for snap operations, besides snap object, Farm has two log files with the below +structure + + struct snap_log { + int epoch; + uint64_t time; + unsigned char sha1[SHA1_LEN]; + }; + +This provides a internal naming mechanism and help us find snap objects by epoch. + +3. VIRTUAL FIGURE + + + sys_snap, user_snap snapshot requests + | | + |put/get snap_sha1 | trigger + v | + +----------+ +------+ +--------+ v +----------+ + | |<------>| snap |<++++++>| | <========> | | + | | +------+ | | | Farm | + | | | trunk | | Working | I/O +-------+ + | |<---------------------->| | | Directory| <~~~~~~>|sheep | + | Farm | +--------+ | | +-------+ + | Store | | | + | | | | + | |<-------------------------------------------->| | + | | | | + +----------+ +----------+ + +<-----> put/get objects to/from Farm Store +<+++++> put/get trunk_sha1 to/from snap object +<=====> put/get oid/oid_sha1 pairs to/from trunk object + -- 1.7.8.rc3 |