Minimum network bandwidth for Mirrordisk/UX?

[Jyotinath Ganguly]

What is the minimum network bandwidth and network type recommended or required to implement Mirrordisk/UX across a WAN?

[A. Clay Stephenson]

Mirror/UX is intended to be used for locally attached disks (copper or Fibre SCSI); your question is a non sequiter.

When looking at long-distance replication, one generally looks ats an embedded technology assicated with a disk array such as SRDF.

[Bill Hassell · Accepted solution]

Mirroring disks across a WAN is incredibly expensive and as Clay mentions, must be handled through expensive disk arrays. The bandwidth is fairly easy to calculate: Modern disks transfer data between 30 and 100 Mbytes/sec so this would be the worst case as far as speed. Here are typical speeds for network cards:

10Base-T = 0.5 Mbytes/sec
100Base-T = 5 Mbytes/sec
1000Base-T = 50 Mbytes/sec

NOTE: LAN speeds are in bits per second. Divide by 10 to get the approximate bytes per second rate. Then divide that number in half to get a typical data rate. Note also that WANs have very, very long response delays so getting an acknowledgement from the mirror side may drop the effective rate to less than 1/3 of the non-WAN speeds.

So ask your local phone company or ISP how much a 1000 to 3000 Mbit WAN Link will cost. Note that if you do not get a fast link, your primary server will be devastated with long delays trying to get the data mirrored through the slow WAN.

[rick jones]

And perhaps even more important than the bandwidth is the latency incurred across the WAN. Particularly if the write to local storage will not be marked as complete until after the remote mirror is written... depending on the distances involved, this can add 30 or more milliseconds to the time it takes to complete an I/O. Being then able to acheive a given bandwidth will require additional parallelism - ie more I/Os in flight at one time.

IIRC there are some schemes which do the mirroring in the background - improvement in perf at the expense of possible data not mirrored when the local fails...