Re: Index Scans become Seq Scans after VACUUM ANALYSE

Michael Loftis wrote:
> A Block-sized read will not be broken up. But if you're reading ina
> size bigger than the underlying systems block sizes then it can get
> broken up.
>
> So yes a sequential read will get broken up. A single read request for
> a block may or may not get broken up. If you're freading with set block
> sizes you'll see the set sizes of blocks come through, but what the
> underlying OS does is undefined, same for writing. If the underlying
> block size is 8KB and you dump 4MB down on it, the OS may (and in many
> cases does) decide to write part of it, do a read ona nearby sector,
> then write the rest. This happens when doing long writes that end up
> spanning block groups because the inodes must be allocated.

Also keep in mind most disks have 512 byte blocks, so even if the file
system is 8k, the disk block sizes are different. A given 8k or 1k file
system block may not even be all in the same cylinder.

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