why does df spin up disks?

why does df spin up disks?

[Russell Coker]

When I have a mounted filesystem why doesn't the kernel store the amount of 
free space?  Why does it need to spin up a disk that had been spun down?

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Re: why does df spin up disks?

[Holger Hoffstätte]

On 06/29/15 03:43, Russell Coker wrote:
> When I have a mounted filesystem why doesn't the kernel store the amount of 
> free space?  Why does it need to spin up a disk that had been spun down?

Most likely because the inode has been evicted due to memory pressure. I can df my mostly-idle backup disk "most" of the time without it spinning up once it has been mounted & gone to sleep (just did!), but if there's been significant memory movement (or issuing drop_caches) on the box it will spin up again sometimes. This is not unique to btrfs; other filesystems - at least ext4 - do this too, even though they might manage their expiry behaviour differently.

Now, whether the root inode and whatever is required for a df *should* ever expire after mounting or stay pinned, well..you'd have to ask the vfs folks.

-h