[OpenAFS] Re: Performance issues

[Jeffrey Altman]

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On 8/24/2011 9:17 AM, Jaap Winius wrote:
> But, wouldn't that cause trouble in the mornings when those workstation=
s
> are turned on again and a number of them try to re-synchronize their 1G=
B
> caches across my relatively puny 6Mbps WAN links?

Caches are not actively sync'd.  Caches are passive data stores that are
used to avoid copying data across the network when the data is known to
already exist locally.  If the required data is not present in the
cache, then it is read from the file server.  If the cache is too small,
more data is read from the file server than would otherwise be necessary.=


Cache coherency is maintained in AFS via the use of status information
which has an expiration time and a callback protocol that is used by the
file server to notify the client when the unexpired status information
has been altered.  AFS does not read and cache whole files. It reads
file chunks which are tracked by File ID, File Offset, and Data Version.
 Data is used from the cache when the File ID, File Offset and Data
Version required to satisfy an application request match.  Otherwise,
the data is read from the file server.

For read/write data, if the cache is too small, the cache manager is
required to flush data to the file server sooner than it would prefer.
Since many files used today are in the GB range, it is not unusual to
have caches sizes of 10GB to 20GB.  The local disk is cheap; network
bandwidth is not.

Jeffrey Altman


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