Question: I have a simple Xsan: two-seats, one Xserve RAID, one storage pool. After copying a whole bunch of tiny files, via the Finder, to the SAN, any write creates a file with at least 30 extents, sometimes as much as 500 on a single file. I tried defragmenting the whole volume, deleting the directories that had the zillion little files, I even upgraded the clients to OS 10.3.9/Xsan 1.0.1. The large-extent render files are not playing back unless I defrag them. My questions are: Are 30-500 extents for single files (100-500MB render files for example) normal? Do you know of anything to cure this poor file system?
Answer: We need to understand several behaviors of Xsan:
- Metadata is stored in blocks contiguous with data
- Metadata is written to disk after every new file created
- Metadata is easy to fragment, and difficult (impossible?) to defragment
- New files use the first available free space on a storage pool
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