What is the meaning of "recall storms" in vfs_gpfs.8?

[Christof Schmitt]

On Mon, Mar 28, 2016 at 10:18:40AM +0200, Michael Adam wrote:
> On 2016-03-28 at 16:11 +0900, Samba-JP oota wrote:
> > In Samba 4.2.0 vfs_gpfs.8 document,
> > Some phrases were added like this.
> > 
> > <para> When this option is set to no, an attempt to
> >        open an offline file will be rejected with access
> >        denied.  This helps preventing recall storms triggered
> >                                       -------------
> >        by careless applications like Finder and
> >        Explorer.</para>
> > 
> > What kind of meaning is "recall storms" in this phrase?
> > 
> > Is Explorer to perform re-reading of a file repeatedly?
> 
> Recall is the process of getting files back from tape
> (or other slower tier in the HSM) back to the faster
> front tier.
> 
> The storm refers to _many_ files being recalled more or
> less simultaneously, triggered by certain actions of
> explorer (etc).
> 
> The problem here is that the explorer always reads the
> first few bytes of a file which is not flagged offline
> in order to get the icon, etc. So the non-offline files
> in the folder need to be recalled for listing.
> 
> Hence it is not necessary to read files repeatedly in
> order to recall _many_ files (create a recall storm):
> Just displaying a directory with a lot of files will do it.
> 
> Hope this makes it more clear!

Just to clarify: Reading from or writing to a file that has been
migrated to tape will trigger the recall to disk.

The main concern are SMB clients that are not aware of the offline flag,
and will read data from all files during a directory listing, thus
triggering the recall of many files.

Christof