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SubjectPROBLEM: nfs & "Warning - running *really* short on DMA buffers"
   Hello to all! While preparing to migrate some servers to Redhat 
7.3 and doing some nfs tests before deployment i came across this
"Warning - running *really* short on DMA buffers" error message
repeated several times in dmesg and /var/log/messages. Something like
this:

Aug 30 07:12:52 TEST kernel: Warning - running *really* short on DMA
buffers
Aug 30 07:12:52 TEST kernel: Warning - running *really* short on DMA
buffers
Aug 30 07:13:55 TEST last message repeated 18 times
Aug 30 07:13:55 TEST kernel: DMA buffers
Aug 30 07:13:55 TEST kernel: Warning - running *really* short on DMA
buffers
Aug 30 07:13:55 TEST last message repeated 320 times


Please note that the "kernel: DMA buffers" line was exactly what
appeared. Sometimes ksyslog seems to eat characters when this
happens. Anyway, the most intriguing part is that while i was testing
with SGI Irix 6.5.13, Solaris 2.7 and Aix 4.3 nfs clients, only the
SGI Irix ones make the problem appear, and only when testing async
nfs v3 writes. In the same Irix machines, if i change the server nfs
export to sync, or i mount the export forcing nfs v2, the problem
doesn't repeat itself. The test i was making was a simple

time dd if=/dev/zero of=/vol01/nfstestrun01 bs=16k count=32768

Both the server and the clients have negotiated 100Mb FullDuplex, and
for the curious of you that wonder if the results of the test are the
same in all client machines, i have to say that at least for async
writes, they are more or less the same. Please observe:

NFS v3 async writes(averages):

Irix 6.5.13 1:15.28 9.8%
Aix 4.3 1:12.47 35.4%
Solaris 2.7 1:13.66 15.6%


NFS v3 sync writes(averages):

Irix 6.5.13 2:14.10 5.2%
Aix 4.3 1:29.46 25.3% (not a typo)
Solaris 2.7 2:21.65 7.0%


I am attaching details on the server configuration, kernel, dmesg and
vmstat output.


Thanks in advance,

Pedro Rodrigues
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Date: 30 Aug 2002, 9:00
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