So, i got persuaded to switch from a “server that is going to do everything” to “compute server + storage server”
The two are connected via a DAC on an intel x520 network card.
Compute is 10.0.0.1, Storage is 10.255.255.254 and i left the usable hosts in the middle for future expansion.
Before I start to use it, I’m wondering if i chose the right protocols to share data between them.
I set NFS and iSCSI.
With iSCSI i create an image, share that image on the compute server, format it as btrfs, use it as a native drive. Files are not accessible anywhere else.
With NFS i just mount the share and files can be accessed from another computer.
Speed:
I tried to time how long it takes to fill a dummy file with zeroes.
/iscsi# time sh -c "dd if=/dev/zero of=ddfile bs=8k count=250000 && sync"
250000+0 records in
250000+0 records out
2048000000 bytes (2.0 GB, 1.9 GiB) copied, 0.88393 s, 2.3 GB/s
real 0m2.796s
user 0m0.051s
sys 0m0.915s
/nfs# time sh -c "dd if=/dev/zero of=ddfile bs=8k count=250000 && sync"
250000+0 records in
250000+0 records out
2048000000 bytes (2.0 GB, 1.9 GiB) copied, 2.41414 s, 848 MB/s
real 0m3.539s
user 0m0.038s
sys 0m1.453s
/sata-smr-wd-green-drive-for-fun# time sh -c "dd if=/dev/zero of=ddfile bs=8k count=250000 && sync"
250000+0 records in
250000+0 records out
2048000000 bytes (2.0 GB, 1.9 GiB) copied, 10.1339 s, 202 MB/s
real 0m46.885s
user 0m0.132s
sys 0m2.423s
what i see from this results:
the sata slow drive goes at 1.6 gigabit/s but then for some reason the computer needs so much time to acknowledge the operation.
nfs transferred it at 6.8 gigabit/s which is what i expected from a nvme array. Same command on the storage server gives similar speed.
iscsi transfers at 18.4 gigabit/s which is not possible with my drives and the fiber connection. Probably is using some native file system trickery to detect “it’s just a file full of zeroes, just tell the user it’s done”
The biggest advantage of NFS is that I can share a whole directory and get direct access. Also sharing another disk image via iscsi requires a service restart which means i have to take down the compute server.
But with iscsi i am the owner of the disk so i can do whatever i want, don’t need to worry about permissions, i am root, chown all the stuff
So… after this long introduction and explanation, what protocol would you use for…:
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/var/lib/mysql - a database. Inside a disk image shared via iscsi or via nfs?
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virtual machine images. Copy them inside another image that’s then shared via iscsi? Maybe nfs is much better for this case. Otherwise with iscsi i would have a single giant disk image that contains other disk images…
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lots of small files like WordPress. Maybe nfs would add too much overhead? But it would be much easier to backup if it was an NFS share instead of a disk image
For a DB, iscsi all day long. At work we have some of the fastest storage available on the market today (SCM drives in front of E1.L super fast SSD all connected with 100gbe mellanox switches and NICs) and it barely can keep up with the MySQL DB. NFS would absolutely work for the other use cases. But these are enterprise class systems for an entire university so yeah lots of load.
For a home system, I’m not sure it matters if the load isn’t much. I prefer iscsi because I know how it works. I use it for VMware datastores even at home. Anything that is shared storage between webservers or the like, NFS is almost required since shared iscsi storage is going to be very difficult to set up outside of something like VMware datastores since the file system is setup for multi-writers. Other filesystems like Redhat’s GFS2 or Oracle’s OCFS can do it but it’s not cheap or easy.
Tl;dr summary: my suggestion is iscsi for the DB and NFS for the other stuff.