On Thu, 2022-09-01 at 14:32 +0100, Daire Byrne wrote: > Hi, > > So I have a bit of a newbie question (apologies) that came to me > while > debugging some code that was spamming our NFS servers with lookups > for > nonexistent files. > > If we can cache directory entries (readdir) and even all their > attributes (readdirplus) for some specified period of time (actimeo, > nocto) on a client, then why can't we use that data to serve negative > lookups for files in that directory too (if we so choose)? > > There are probably very good reasons you always need to do a > (negative) file lookup, like being able to read files recently > created > on another client (despite your local cache for that directory), but > I'm just curious what the official reasons are. If we could choose to > serve negative lookups using the directory entries cache for a > read-only or unchanging filesystem, would that still be bad? We > already choose to use nocto for some workloads... > > In our case we see these kinds of heavy negative lookup workloads for > network installed software (100 entries in PYTHONPATH is bad) and in > buggy software (randomly generated filename lookups are really bad!). > Of course, this overhead gets really bad as you add latency between > the client and server. > > Daire man 5 nfs Look for the section on the 'lookupcache=mode' mount option. -- Trond Myklebust Linux NFS client maintainer, Hammerspace trond.myklebust@xxxxxxxxxxxxxxx
