High availability email server...

David S. Madole david at madole.net
Fri Jul 28 15:28:00 EDT 2006

> From: Pascal Gienger
> The capability to handle small files efficiently is related to the 
> filesystem carrying the files and NOT to the physical and logical 
> storage media (block device) under it.
> A SAN is a network where physical and logical block devices are shared 
> between nodes and which makes it possible to mount a harddisk or raid 
> partition as a block device even if the disk is some miles away - from 
> this point of view there is no difference between iSCSI (IP) and 
> FiberChannel
> (FCP) besides different hardware.
> For your host/mail server there is no difference between having the 
> filesystem on - say - a 300 GB local hard drive partition or having it 
> on a SAN volume routed between SAN switches. The OS "sees" it like a 
> normal block device, and the filesystem just uses that.
> So if Apple says that Xsan does not handle many files they admit that 
> their
> HFS+ file system is crap for many small files.

That's just not true as a general statement. SAN is a broad term that applies to much more than just farming out block devices. Some of the more sophisticated SANs are filesystem-based, not block-based. This allows them to implement more advanced functionality like cross-platform sharing of volumes, simultaneous mounts of volumes from different hosts, backups (and single-file restores) performed by the SAN system, pooling of free space, transparent migration to offline storage, etc., etc., etc.

Although I have no experience with it, it appears from a quick check of the web that Apple's XSAN indeed works that way and does not use HFS. So it's entirely credible for them to say that it is supported on a local filesystem but not XSAN, they are two different filesystems, not just different block devices. However, if you want to make a statement that the XSAN filesystem is crap for many small files, then that might be justified.

Another instance of a SAN filesystem that I do happen to be familiar with is IBM's:



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