Add instructions to change NFS_MAX_FILE_IO_SIZE

docs/usage/nfs.en.md

@@ -14,6 +14,9 @@ Commands:
 - [upgrade](#upgrade)
 - [defrag](#defrag)
 
+⚠️ Important: follow the instructions from [Linux NFS write size](#linux-nfs-write-size)
+for optimal Vitastor NFS performance if you use EC and HDD and mount your NFS from Linux.
+
 ## Pseudo-FS
 
 Simplified pseudo-FS proxy is used for file-based image access emulation. It's not
@@ -100,6 +103,62 @@ Other notable missing features which should be addressed in the future:
   in the DB. The FS is implemented is such way that this garbage doesn't affect its
   function, but having a tool to clean it up still seems a right thing to do.
 
+## Linux NFS write size
+
+Linux NFS client (nfs/nfsv3/nfsv4 kernel modules) has a hard-coded maximum I/O size,
+currently set to 1 MB - see `rsize` and `wsize` in [man 5 nfs](https://linux.die.net/man/5/nfs).
+
+This means that when you write to a file in an FS mounted over NFS, the maximum write
+request size is 1 MB, even in the O_DIRECT mode and even if the original write request
+is larger.
+
+However, for optimal linear write performance in Vitastor EC (erasure-coded) pools,
+the size of write requests should be a multiple of [block_size](../config/layout-cluster.en.md#block_size),
+multiplied by the data chunk count of the pool ([pg_size](../config/pool.en.md#pg_size)-[parity_chunks](../config/pool.en.md#parity_chunks)).
+When write requests are smaller or not a multiple of this number, Vitastor has to first
+read paired data blocks from disks, calculate new parity blocks and only then write them
+back. Obviously this is 2-3 times slower than a simple disk write.
+
+Vitastor HDD setups use 1 MB block_size by default. So, for optimal performance, if
+you use EC 2+1 and HDD, you need your NFS client to send 2 MB write requests, if you
+use EC 4+1 - 4 MB and so on.
+
+But Linux NFS client only writes in 1 MB chunks. 😢
+
+The good news is that you can fix it by rebuilding Linux NFS kernel modules 😉 🤩!
+You need to change NFS_MAX_FILE_IO_SIZE in nfs_xdr.h and then rebuild and reload modules.
+
+The instruction, using Debian as an example (should be ran under root):
+
+```
+# download current Linux kernel headers required to build modules
+apt-get install linux-headers-`uname -r`
+
+# replace NFS_MAX_FILE_IO_SIZE with a desired number (here it's 4194304 - 4 MB)
+sed -i 's/NFS_MAX_FILE_IO_SIZE\s*.*/NFS_MAX_FILE_IO_SIZE\t(4194304U)/' /lib/modules/`uname -r`/source/include/linux/nfs_xdr.h
+
+# download current Linux kernel source
+mkdir linux_src
+cd linux_src
+apt-get source linux-image-`uname -r`-unsigned
+
+# build NFS modules
+cd linux-*/fs/nfs
+make -C /lib/modules/`uname -r`/build M=$PWD -j8 modules
+make -C /lib/modules/`uname -r`/build M=$PWD modules_install
+
+# move default NFS modules away
+mv /lib/modules/`uname -r`/kernel/fs/nfs ~/nfs_orig_`uname -r`
+depmod -a
+
+# unload old modules and load the new ones
+rmmod nfsv3 nfs
+modprobe nfsv3
+```
+
+After these (not much complicated 🙂) manipulations NFS begins to be mounted
+with new wsize and rsize by default and it fixes Vitastor-NFS linear write performance.
+
 ## Horizontal scaling
 
 Linux NFS 3.0 client doesn't support built-in scaling or failover, i.e. you can't