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[Linux-cluster] performance bottleneck on 36-disk GFS/NFS cluster



We have a 2.5 TB GFS (6.1) with 2 TB of data spread via RAID10 metaLUNs on an EMC CX500. The GFS is running on 4 nodes of varying size (some have 24CPU 2GB RAM. others 2CPU 1GB RAM), exported via NFS to 10 NFS clients which are POP/IMAP/SMTP servers in an ISP environment. The IPs are managed by rgmanager.

The data is a couple of hundred thousand mailboxes (in MailDir) format.

Some performance metrics:
- Load average for NFS servers is about 8 - 16 per NFS client mounted. The big servers have 4 clients each (32 - 64 load average). The smaller servers have 1 client each (8 - 16 load average). - dlm_recvd is by far the busiest process in top (10 - 20%), followed by nfsd processes and gfs_inoded, lock_dlm, gfs_scand.

Here is the output of one of the outputs of "iostat -x dm-0 5":

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s rkB/s wkB/s avgrq-sz avgqu-sz await svctm %util dm-1 0.00 0.00 2043.23 970.91 16345.86 7767.27 8172.93 3883.64 8.00 35.62 11.86 0.34 101.01 Some notable numbers are 101.01 % utilization, reads per sec and writes per sec in the thousands.

My questions are:
1 (Taking a guestimate) Is my problem lack of spindles, or the inefficiencies of NFS via GFS (I have heard a number of others on the list complain often about NFS on GFS performance).

2 What would be the best way to improve performance? We have a couple of options: a) Collapse/remove the NFS layer. Make a large number of mail servers SAN-attached GFS nodes. (perhaps not all, but 4 to 6. by converting the GFS nodes into mail server) b) add more spindles. We are in the process of adding 24 more spindles (the CX is already taking almost a day to restripe the metaLUN. We might be able to add 24 more spindles and restripe again.

3 Is having dlm_recvd as the top process normal/typical for an I/O bound GFS cluster? Even though the MailDir mail store consists of million of files, nodes should very rarely write into the same directory at the same time (meaning that directory lock contention should be avoided)

thank you in advance
Riaan
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