TableOfContents

General concepts

Working with an AFS+OSD cell

The following techniques are only useful when using OSD as a frontend for HSM:

• {{{vos listobj

}}} is useful for finding all objects stored on a given OSD

• {{{fs prefetch

}}} allows the user to schedule tape restore operations

• it is useful to have bosserver run a script to wipe objects off the OSDs according to OSD fill rate etc.

These are general techniques:

• {{{osd addserver

}}} makes an AFS fileserver known to the OSDDB

• this is required in order to make use of ownerships and locations

How to migrate data from an OSD

1. set a low write priority to stop fileservers from storing data on the OSD in question {{{osd setosd -wrprior 0

}}}

1. use {{{vos listobj

}}} to identify the files (by fid) that have data on the OSD

1. use {{{fs replaceosd

}}} to move each file's data to another OSD

Priorities and choice of storing OSD

• OSDs are used in a Round Robin fashion
• priorities add weight to each OSD
• static priorities can be set by the administrator {{{osd setosd -wrprior ... -rdprior ...

}}}

• priorities are dynamic
  • ownerships apply a modifier of +/- 10 to the static priority
  • locations apply a modifier of +/- 20

Data held in volumes, DBs etc.

• all metadata belonging to files in a volume are stored in a designated volume special file

• metadata references OSDs by id

• OSD id is an index into the OSDDB

  • these IDs are permanent and can never be reused after deletion of an OSDDB entry
  • view the OSDDB using {{{osd l

}}}

• a file's metadata is found using the metadataindex stored in the file's vnode

  • view the metadata using {{{fs osd <file>

}}}

Policies

• policies are going to decide 4 basic things:
  1. whether object storage is to be used for a given file
  2. whether the objects are to be mirrored and how often
  3. how many stripes the file will comprise
  4. the size of the stripes
• two pieces of information will be used to make decisions:
  1. file size
  2. file name (i.e. prefix and suffix)

Open questions

• will inheritance be supported for policy definitions?

  • how many levels of inheritance are permitted (1 or n)?

• in what way can policies be represented/stored?

Technical aspects

Performance

• client connections to OSDs are cheap because they are unauthorized
• clients do connect to each single OSD they have business with

• osd psread acts as though reading stripes from multiple OSDs

  • i.e. it opens several connections, in this case all targets are the same however
  • each stripe has an altered offset (e.g., normally first n stripes start at offset 0 on each OSD, here it is 0+(i*stripesize) etc.)
  • this is not impossible for access to AFS fileservers, but making connections is more costly

Notes on the code (changes)

• original linktable was insufficient, further fields were required
  • link table v2 uses 30 bits instead of 15
• hashing is used for management of DB contents

• choice of OSDs resides in osddbuser.c

Debugging techniques

• add trace* calls to the code
  • CM_TRACE_WASHERE especially handy

• use