Path of a disk IO
Hi all,
I know this is a pretty big question but just trying to learn as much as I can about this topic.
I wanted to understand how IO travels in a SAN. Lets say for example, I have a linux host connected to a SAN.
If I copy a 2MB file, can someone walk me through what happens. I realize a LOT happens but looking for any info you can provide (which will probably lead to me having more questions to look up).
I'm looking for the path from host, to cache, to HBA card, over the fiber channel, to the SAN HBA, disk cache, and onto disk. (and if I missed anything).
I'm not looking for SAN vendor specifics, just more of a general flow.
For example, is that 2MB file broken up into 4K IO's (524,288 of them??) and are they all sent seperately over the fiber channel...etc..etc..
Any help or pointers you could provide would be greatly appreciated.
thanks in advance!
Software/Hardware used:
Software/Hardware used:
ASKED:
June 20, 2009 3:53 PM
UPDATED:
August 4, 2009 4:41 PM
Answer Wiki:
As I understand it, here's how it works.
You write the file. The file system is set for say 4k blocks so the OS and the file system break the file into 4k chunks. Those are sent to the HBA, which passes the data through the fibre switch to the SAN array. From there it's written to cache at which point the host assumes that the data has been saved to disk.
The SAN will keep the data in cache until it's been idle for <i>X</i> amount of time or the cache fills to <i>N</i>% (where <i>X</i> and <i>N</i> are defined by the SAN admin or the SAN vendor). When this happens the SAN will destage the data from the cache to the physical disk.
If the SAN is configured for a block size smaller than what the OS is configured for then when it's written to cache the SAN breaks the 4k blocks into the smaller size. If the SAN is configured for a larger block size then larger blocks are allocated.
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