Hi,
I'm trying to record the full X310 bandwidth, for a few hours, without
any missed samples. Which of course is a bit of a challenge - does
anyone here already achieve this?
We're using a TwinRX, so initially I wanted to record 2x 100MS/s (from
both channels), which amounts to 800MB/s, 6.4Gb/s. At first I tried
uhd_rx_cfile, but have been unable to get it to a good state without
showing an 'O' every few seconds at these speeds.
As a recorder I have a SuperMicro 847 chassis with 36 disks (Seagate
Ironwolf 8TB T8000VN0022, 7200rpm). In this particular server, the disks
are connected through an 'expander' backplane, from a single HBA (LSI
3008). CPU is dual Xeon 4110, 2.1 GHz, 64 GB of ram.
At first I tried a 6 disk pool (raidz1), and eventually ended up
creating a huge 36 disk ZFS stripe, which in theory should have no
trouble with the throughput, but certainly kept dropping packets.
Note that recording to /dev/shm/file works perfectly without dropping
packets, until the point that the memory is full.
Given that ZFS has quite a bit of (good) overhead to safeguard your
data, I then switched to creating a mdadm raid-0 with 18 of the disks
(Why not 36? I was really running out of time!)
At that point I also found 'specrec' from gr-analyze, which seems more
suitable. But, even after enlarging its circular buffer to the largest
supported values, it would only average a write speed of about 300MB/s.
In the end I had to settle for recording at only 50MS/s (200MB/s) from
only a single channel, a far cry from the 2x 6.4Gb/s I'm ultimately
looking to record. Although I did get more than an hour of perfect data
out of it, over time the circular buffer did get fuller in bursts, and
within 2 hours it exited due to exhausting the buffers. Restarting the
application made it work like fresh again, with the same gradual decline
in performance.
Specrec, even when tweaking its settings, doesn't really take advantage
of the large amount of memory in the server. As a next step, I'm
thinking of adapting specrec to use much larger buffers, so that writes
are at least in the range of MB to tens of MB. From earlier experiences,
it is also important to flush your data to disk often, so the
interruptions due to this are more frequent, but short enough to not
cause receive buffers to overflow.
In terms of network tuning, all recording was done with MTU 9000, with
wmem and rmem at the recommended values. All recordings were done as
interleaved shorts.
Does anyone have hints or experiences to share?
Regards, Paul Boven.
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