[mythtv-users] How many simultaneous recordings on one drive?

[Martin Bene]

> What is the limit (realistically) for one drive I/O?  I don't run jobs during primetime; from 4pm-11pm the machine has no jobs.
> If it can't, should I just get an hdhomerun single (cheap on the used market)... or am I already pushing it, performance wise?

Let's just throw a simple benchmark at that question and see what happens.

* You've already got the OS/DB on a separate drive - good, that makes testing much easier.
* you're not interested in the added complication of  recording + running jobs at the same time.

I'll just use tiobench, which concentrates on testing with multiple concurrent threads. What I'm most interested in is throughput of concurrent sequential writes, which corresponds to recording multiple streams.

Memory size for the tests was 256M, tests were run with a 1GB file size, so tests should reflect the actual DISK I/O. Test was run with 1/2/4/8/16/32/64 threads, representing concurrent recordings. If we estimate  2MB/s for each recorded stream we net a total throughput in MB/s

Num streams     required throughput
==========      ================
1               2
2               4
4               8
8               16
16              32
32              64

Now let's look at the benchmarks for a single 1.5TB disk (Seagate, 7200K, ST31500341AS)

                              File  Blk   Num                   Avg      Maximum      Lat%     Lat%    CPU
Identifier                    Size  Size  Thr   Rate  (CPU%)  Latency    Latency      >2s      >10s    Eff
---------------------------- ------ ----- ---  ------ ------ --------- -----------  -------- -------- -----
single_1.5                    1000  4096     1   83.51 15.23%     0.046      220.90   0.00000  0.00000   548
single_1.5                    1000  4096     2   79.82 29.85%     0.094      621.05   0.00000  0.00000   267
single_1.5                    1000  4096     4   77.60 61.31%     0.190      925.87   0.00000  0.00000   127
single_1.5                    1000  4096     8   71.53 110.5%     0.389     1829.04   0.00000  0.00000    65
single_1.5                    1000  4096   16   71.43 220.3%     0.757     2624.33   0.00158  0.00000    32
single_1.5                    1000  4096   32   64.37 396.1%     1.570     3925.16   0.01181  0.00000    16
single_1.5                    1000  4096   64   57.86 726.1%     3.128     4784.39   0.04150  0.00000     8

So, just for basic recording of  HD streams, no jobs, no frontends playing the recording bandwidth of our single disk gave out at about 32 HD recordings. Obviously, this is just a benchmark and not a real life situation - You'd probably run into issues with a lot fewer recordings (max latency for example), but the benchmark does give you a baseline to work from.

Adding a hdhomerun should not push your disk beyond its limits.

Bye, Martin


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