----- On 12 Nov, 2020, at 13:01, Daire Byrne [email protected] wrote:
>
> Having just completed a bunch of fresh cloud rendering with v5.9.1 and Trond's
> NFSv3 lookupp emulation patches, I can now revise my original list of issues
> that others will likely experience if they ever try to do this craziness:
>
> 1) Don't re-export NFSv4.0 unless you set vfs_cache_presure=0 otherwise you
> will
> see random input/output errors on your clients when things are dropped out of
> the cache. In the end we gave up on using NFSv4.0 with our Netapps because the
> 7-mode implementation seemed a bit flakey with modern Linux clients (Linux
> NFSv4.2 servers on the other hand have been rock solid). We now use NFSv3 with
> Trond's lookupp emulation patches instead.
>
> 2) In order to better utilise the re-export server's client cache when
> re-exporting an NFSv3 server (using either NFSv3 or NFSv4), we still need to
> use the horrible inode_peek_iversion_raw hack to maintain good metadata
> performance for large numbers of clients. Otherwise each re-export server's
> clients can cause invalidation of the re-export server client cache. Once you
> have hundreds of clients they all combine to constantly invalidate the cache
> resulting in an order of magnitude slower metadata performance. If you are
> re-exporting an NFSv4.x server (with either NFSv3 or NFSv4.x) this hack is not
> required.
>
> 3) For some reason, when a 1MB read call arrives at the re-export server from
> a
> client, it gets chopped up into 128k read calls that are issued back to the
> originating server despite rsize/wsize=1MB on all mounts. This results in a
> noticeable increase in rpc chatter for large reads. Writes on the other hand
> retain their 1MB size from client to re-export server and back to the
> originating server. I am using nconnect but I doubt that is related.
>
> 4) After some random time, the cachefilesd userspace daemon stops culling old
> data from an fscache disk storage. I thought it was to do with setting
> vfs_cache_pressure=0 but even with it set to the default 100 it just randomly
> decides to stop culling and never comes back to life until restarted or
> rebooted. Perhaps the fscache/cachefilesd rewrite that David Howells & David
> Wysochanski have been working on will improve matters.
>
> 5) It's still really hard to cache nfs client metadata for any definitive time
> (actimeo,nocto) due to the pagecache churn that reads cause. If all required
> metadata (i.e. directory contents) could either be locally cached to disk or
> the inode cache rather than pagecache then maybe we would have more control
> over the actual cache times we are comfortable with for our workloads. This
> has
> little to do with re-exporting and is just a general NFS performance over the
> WAN thing. I'm very interested to see how Trond's recent patches to improve
> readdir performance might at least help re-populate the dropped cached
> metadata
> more efficiently over the WAN.
>
> I just want to finish with one more crazy thing we have been doing - a
> re-export
> server of a re-export server! Again, a locking and consistency nightmare so
> only possible for very specific workloads (like ours). The advantage of this
> topology is that you can pull all your data over the WAN once (e.g. on-premise
> to cloud) and then fan-out that data to multiple other NFS re-export servers
> in
> the cloud to improve the aggregate performance to many clients. This avoids
> having multiple re-export servers all needing to pull the same data across the
> WAN.
I will officially add another point to the wishlist that I mentioned in Bruce's
recent patches thread (for dealing with the iversion change on NFS re-export).
I had held off mentioning this one because I wasn't really sure if it was just
a normal production workload and expected behaviour for NFS, but the more I
look into it, the more it seems like maybe it could be optimised for the
re-export case. But then I also might be too overly sensitive about metadata
ops over the WAN at this point....
6) I see many fast repeating COMMITs & GETATTRs from the NFS re-export server
to the originating server for the same file while writing through it from a
client. If I do a write from userspace on the re-export server directly to the
client mountpoint (i.e. no re-exporting) I do not see the GETATTRs or COMMITs.
I see something similar with both a re-export of a NFSv3 originating server and
a re-export of a NFSv4.2 originating server (using either NFSv3 or NFSv4).
Bruce mentioned an extra GETATTR in the NFSv4.2 re-export case for a COMMIT
(pre/post).
For simplicity let's look at the NFSv3 re-export of an NFSv3 originating
server. But first let's write a file from userspace directly on the re-export
server back to the originating server mount point (ie no re-export):
3 0.772902 V3 GETATTR Call, FH: 0x6791bc70
6 0.781239 V3 SETATTR Call, FH: 0x6791bc70
3286 0.919601 V3 WRITE Call, FH: 0x6791bc70 Offset: 1048576 Len: 1048576
UNSTABLE [TCP segment of a reassembled PDU]
3494 0.921351 V3 WRITE Call, FH: 0x6791bc70 Offset: 8388608 Len: 1048576
UNSTABLE [TCP segment of a reassembled PDU]
...
...
48178 1.462670 V3 WRITE Call, FH: 0x6791bc70 Offset: 102760448 Len: 1048576
UNSTABLE
48210 1.472400 V3 COMMIT Call, FH: 0x6791bc70
So lots of uninterrupted 1MB write calls back to the originating server as
expected with a final COMMIT (good). We can also set nconnect=16 back to the
originating server and get the same trace but with the write packets going down
different ports (also good).
Now let's do the same write through the re-export server from a client (NFSv4.2
or NFSv3, it doesn't matter much):
7 0.034411 V3 SETATTR Call, FH: 0x364ced2c
286 0.148066 V3 WRITE Call, FH: 0x364ced2c Offset: 0 Len: 1048576 UNSTABLE
[TCP segment of a reassembled PDU]
343 0.152644 V3 WRITE Call, FH: 0x364ced2c Offset: 1048576 Len: 196608
UNSTABLEV3 WRITE Call, FH: 0x364ced2c Offset: 1245184 Len: 8192 FILE_SYNC
580 0.168159 V3 WRITE Call, FH: 0x364ced2c Offset: 1253376 Len: 843776
UNSTABLE
671 0.174668 V3 COMMIT Call, FH: 0x364ced2c
1105 0.193805 V3 COMMIT Call, FH: 0x364ced2c
1123 0.201570 V3 WRITE Call, FH: 0x364ced2c Offset: 2097152 Len: 1048576
UNSTABLE [TCP segment of a reassembled PDU]
1592 0.242259 V3 WRITE Call, FH: 0x364ced2c Offset: 3145728 Len: 1048576
UNSTABLE
...
...
54571 3.668028 V3 WRITE Call, FH: 0x364ced2c Offset: 102760448 Len: 1048576
FILE_SYNC [TCP segment of a reassembled PDU]
54940 3.713392 V3 WRITE Call, FH: 0x364ced2c Offset: 103809024 Len: 1048576
UNSTABLE
55706 3.733284 V3 COMMIT Call, FH: 0x364ced2c
So now we have lots of pairs of COMMIT calls inbetween the WRITE calls. We also
see sporadic FILE_SYNC write calls which we don't when we just write direct to
the originating server from userspace (all UNSTABLE).
Finally, if we add nconnect=16 when mounting the originating server (useful for
increasing WAN throughput) and again write through from the client, we start to
see lots of GETATTRs mixed with the WRITEs & COMMITs:
84 0.075830 V3 SETATTR Call, FH: 0x0e9698e8
608 0.201944 V3 WRITE Call, FH: 0x0e9698e8 Offset: 0 Len: 1048576 UNSTABLE
857 0.218760 V3 COMMIT Call, FH: 0x0e9698e8
968 0.231706 V3 WRITE Call, FH: 0x0e9698e8 Offset: 1048576 Len: 1048576
UNSTABLE
1042 0.246934 V3 COMMIT Call, FH: 0x0e9698e8
...
...
43754 3.033689 V3 WRITE Call, FH: 0x0e9698e8 Offset: 100663296 Len: 1048576
UNSTABLE
44085 3.044767 V3 COMMIT Call, FH: 0x0e9698e8
44086 3.044959 V3 GETATTR Call, FH: 0x0e9698e8
44087 3.044964 V3 GETATTR Call, FH: 0x0e9698e8
44088 3.044983 V3 COMMIT Call, FH: 0x0e9698e8
44615 3.079491 V3 WRITE Call, FH: 0x0e9698e8 Offset: 102760448 Len: 1048576
UNSTABLE
44700 3.082909 V3 WRITE Call, FH: 0x0e9698e8 Offset: 103809024 Len: 1048576
UNSTABLE
44978 3.092010 V3 COMMIT Call, FH: 0x0e9698e8
44982 3.092943 V3 COMMIT Call, FH: 0x0e9698e8
Sometimes I have seen clusters of 16 GETATTRs for the same file on the wire
with nothing else inbetween. So if the re-export server is the only "client"
writing these files to the originating server, why do we need to do so many
repeat GETATTR calls when using nconnect>1? And why are the COMMIT calls
required when the writes are coming via nfsd but not from userspace on the
re-export server? Is that due to some sort of memory pressure or locking?
I picked the NFSv3 originating server case because my head starts to hurt
tracking the equivalent packets, stateids and compound calls with NFSv4. But I
think it's mostly the same for NFSv4. The writes through the re-export server
lead to lots of COMMITs and (double) GETATTRs but using nconnect>1 at least
doesn't seem to make it any worse like it does for NFSv3.
But maybe you actually want all the extra COMMITs to help better guarantee your
writes when putting a re-export server in the way? Perhaps all of this is by
design...
Daire
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