Browse Prior Art Database

Token Synchronized I/O

IP.com Disclosure Number: IPCOM000110005D
Original Publication Date: 1992-Oct-01
Included in the Prior Art Database: 2005-Mar-25
Document File: 3 page(s) / 131K

Publishing Venue

IBM

Related People

Heyrman, PJ: AUTHOR [+4]

Abstract

Described is a method for serializing access to point-in-time snapshot copies of disk drive (DASD) sectors, where the snapshot copy of a sector is an image made during the first write request to the sector after a checkpoint state has been established and which contains the contents of the sector at the time of the checkpoint. This method allows the write of the snapshot copy of a sector to proceed in parallel with the write of the original sector, and also allows the point-in-time image of the data to be accurately reconstructed at any time even if the state of the data is in flux. Tokens logically associated with the original DASD sectors are used to serialize read/write access to the snapshot copies.

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This is the abbreviated version, containing approximately 47% of the total text.

Token Synchronized I/O

       Described is a method for serializing access to
point-in-time snapshot copies of disk drive (DASD) sectors, where the
snapshot copy of a sector is an image made during the first write
request to the sector after a checkpoint state has been established
and which contains the contents of the sector at the time of the
checkpoint.  This method allows the write of the snapshot copy of a
sector to proceed in parallel with the write of the original sector,
and also allows the point-in-time image of the data to be accurately
reconstructed at any time even if the state of the data is in flux.
Tokens logically associated with the original DASD sectors are used
to serialize read/write access to the snapshot copies.

      In order to maintain a snapshot view of the data, it is first
necessary to establish the state of the data which is to be
preserved.  Part of the establishment of this checkpoint state is to
create a number of tokens and a mapping (such as a directory) so that
each sector maps to one and only one token and, given the address of
a sector, the token corresponding to that sector can be located.

      Once the checkpoint has been established, the tokens are used
to synchronize multiple requests to read or write the snapshot data
for a particular sector (or range of sectors).  In particular, the
following situations require synchronization:
o    Since the writes of the original data and the snapshot copies
are allowed to occur in parallel, it is possible for two writes of a
sector to occur such that the first write completes and the second is
requested before the write of the snapshot copy (generated by the
first write request) completes.  To preserve snapshot integrity, it
is necessary to synchronize multiple write requests to the same
sector so that a snapshot copy is generated only by the first write
request made after the checkpoint is established.
o    Since requests to reconstruct the snapshot view of the data may
be made at any time, it is possible that a request to retrieve the
snapshot view of a sector is made when a write of the sector has
completed but the write of the snapshot copy is incomplete.  To
preserve the integrity of the snapshot view of the data, it is
necessary to synchronize the retrieve request with any outstanding
writes so that the correct version of the data can be returned.

      Prior to issuing reads or writes of snapshot data for a
particular sector range, all tokens corresponding to that range must
first be obtained so that the tokens are "owned" by the task wishing
to read or write the snapshot data.  The task then performs the
desired I/O operations against the snapshot data, and releases the
tokens when the I/O is complete.  If one or more tokens are owned by
other tasks when control is requested, the requesting task will wait
until all tokens have been released by the other tasks before
resuming.

      A single token can control access t...