Hi all, I'm trying to investigate tar speeds over NFS. I normally test tar speeds by 'tar cf /dev/null somefilesystem' but if the filesystem is an NFS one, using /dev/null as the output file is resulting in impossible tar speeds. If the tar output is directed to a file rather than to /dev/null, all is well. Does tar do some sort of magic if its tarring NFS stuff to /dev/null? For example: [root@ls1 ~]$ uname -a Linux ls1.lmb.internal 2.6.10-1.770_FC3smp #1 SMP Thu Feb 24 18:36:43 EST 2005 x86_64 x86_64 x86_64 GNU/Linux [root@ls1 ~]$ cat /etc/redhat-release Fedora Core release 3 (Heidelberg) [root@server1 ~]# uname -a Linux server1.mrc-lmb.cam.ac.uk 2.4.7-10 #1 Thu Sep 6 17:21:28 EDT 2001 i586 unknown [root@server1 ~]# cat /etc/redhat-release Red Hat Linux release 7.2 (Enigma) ls1 and server1 are connected by a 100Mbit/sec link. [root@ls1 ~]$ mount /dev/sda7 on / type ext3 (rw) none on /proc type proc (rw) none on /sys type sysfs (rw) none on /dev/pts type devpts (rw,gid=5,mode=620) usbfs on /proc/bus/usb type usbfs (rw) /dev/sda1 on /boot type ext3 (rw) none on /dev/shm type tmpfs (rw) /dev/sda8 on /home type ext3 (rw) /dev/sda2 on /usr type ext3 (rw) /dev/sda5 on /usr/local type ext3 (rw) /dev/sda6 on /var type ext3 (rw) /dev/sdc1 on /tmp type ext3 (rw) none on /proc/sys/fs/binfmt_misc type binfmt_misc (rw) sunrpc on /var/lib/nfs/rpc_pipefs type rpc_pipefs (rw) nfsd on /proc/fs/nfsd type nfsd (rw) /dev/sdd1 on /testnfs type ext3 (rw) automount(pid5393) on /nfs/alf1 type autofs (rw,fd=4,pgrp=5393,minproto=2,maxproto=4) automount(pid5426) on /nfs/server1 type autofs (rw,fd=4,pgrp=5426,minproto=2,maxproto=4) server1:/www on /nfs/server1/www type nfs (rw,addr=10.1.0.0) [root@ls1 ~]$ df -k /testnfs Filesystem 1K-blocks Used Available Use% Mounted on /dev/sdd1 35280616 81840 33406624 1% /testnfs [root@ls1 ~]$ df -k /nfs/server1/www Filesystem 1K-blocks Used Available Use% Mounted on server1:/www 35278544 11907904 21578592 36% /nfs/server1/www [root@ls1 ~]$ time tar cf /testnfs/www.tar -C /nfs/server1/www . 1.164u 60.992s 55:30.78 1.8% 0+0k 0+0io 2pf+0w [root@ls1 ~]$ ls -l /testnfs/www.tar -rw-r--r-- 1 root root 12025507840 May 4 17:38 /testnfs/www.tar The above looks sensible, and produces an archive file with a size as expected. [root@ls1 ~]$ time tar cf /dev/null -C /nfs/server1/www . 0.272u 1.838s 0:23.34 8.9% 0+0k 0+0io 0pf+0w 23 seconds to repeat the tar into /dev/null is impossible if the filesystem really is being read. Just to make sure its not lying around in cache (not that we could fit 12GBytes into 4GByte of memory): [root@ls1 ~]$ umount /nfs/server1/www (The automounter will remount it) [root@ls1 ~]$ time tar cf /dev/null -C /nfs/server1/www . 0.288u 1.861s 0:08.23 26.0% 0+0k 0+0io 0pf+0w And this time its even faster. What is going on here? Any ideas anyone? Cheers, Terry.
