[OpenAFS] poor out of cache behavior on writing

[Paul Blackburn]

Derrick J Brashear wrote:

>On Mon, 17 Feb 2003, Dexter "Kim" Kimball wrote:
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>>It seems that cache garbage collection has some limitations when asked to
>>fill/purge in a short cycle.
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>>By the way, if you're going to routinely be reading/writing large files
>>sequentially, you may want to experiment with AFS cache chunk size.
>>
>>WRT AFS vs. NFS, I believe that in 1) a local area network with 2) a lot of
>>write activity, the caching overhead of AFS is a liability.  (Write to local
>>disk cache, read from local disk cache, network to fileserver, write to
>>fileserver disk.)
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>I think there are cases where it makes sense to bypass the cache (e.g. be
>NFS). I don't know how we might actually figure out how to do so without
>user intervention.
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I agree with Derrick, if we are looking for optimal write performance we are
going to forget distributed filesystems and look at writing to local disk.

You probably will get better data transfer from ftp-client => ftp-server
than a distributed filesystem.

All that said, I suggest you look at the output from "ifconfig".
Nearly every system I have looked at "ifconfig" has far more
Rx (Read) data traffic than Tx (Transmit) data traffic.

Here is a random example:

$ ifconfig tr0
tr0       Link encap:16/4 Mbps Token Ring (New)  HWaddr 00:60:94:55:11:97
          inet addr:10.10.10.19  Bcast:10.10.10.127  Mask:255.255.255.128
          UP BROADCAST RUNNING MULTICAST  MTU:4056  Metric:1
          RX packets:260154 errors:0 dropped:0 overruns:0 frame:0
          TX packets:80397 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100
          RX bytes:229960399 (219.3 Mb)  TX bytes:9374312 (8.9 Mb)
          Interrupt:15 Base address:0x7800

Just to emphasize the point: 219 MB data read and 9 mb data written.
That's about 24 to 1 ratio of read/write data traffic.

In general, for most client systems, there is more data reading than 
data writing.
So, distributed filesystems had better be good at handling read data.
That's one of the things AFS is good at because if its disk (or RAM) cache.

When it comes to "uploading" large datafiles from machine to machine,
I now prefer using SSH's scp which shows me a progress bar and ETA.

For most all other tasks, I like AFS which gives me other capabilities
unheard of in NFS.
--
cheers
paul                                       http://acm.org/~mpb



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