Re: [PATCH RFC 2/2] block: Do not discard buffers under a mounted filesystem

Al Viro
2020-08-27T21:39:07Z

On Thu, Aug 27, 2020 at 08:16:03AM +0100, Christoph Hellwig wrote:
> On Tue, Aug 25, 2020 at 04:50:56PM +0200, Jan Kara wrote:
> > Do you mean that address_space filesystem uses to access its metadata on
> > /dev/sda will be different from the address_space you will see when reading
> > say /dev/sda? Thus these will be completely separate (and incoherent)
> > caches?
>
> Yes.
>
> > Although this would be simple it will break userspace I'm afraid.
> > There are situations where tools read e.g. superblock of a mounted
> > filesystem from the block device and rely on the data to be reasonably
> > recent. Even worse e.g. tune2fs or e2fsck can *modify* superblock of a
> > mounted filesystem through the block device (e.g. to set 'fsck after X
> > mounts' fields and similar).
>
> We've not had any problems when XFS stopped using the block device
> address space 9.5 years ago.

How much writes from fsck use does xfs see, again?