Delaying tape flush ?

I noticed that my LTO-2 tape drive was "starving" for data and shoe-shinning.

I was dumping to tape at 12Mb/sec and the minimum speed to prevent shoe-shinning on an LTO-2 drive is 19Mb/sec.

So I did a check by timing a 'dd' of 10 Gb to the tape drive. It was indeed topped at 12 Mb/sec. It took 13m51s

Now, I replaced the antedeluvian adaptec 2940 with an adaptec 29160 ultra 160 car and redid the exercice.

This time, it took 6m51s, so I was running at about 24 Mb/Sec. That was ok. Not very fast, bot OK.

Now, when I looked at my amanda log this morning, I was very sad to see the following:

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STATISTICS:
                          Total       Full      Incr.
                        --------   --------   --------
Estimate Time (hrs:min)    0:06
Run Time (hrs:min)         2:03
Dump Time (hrs:min)        4:32       2:49       1:44
Output Size (meg)       12963.4     9414.1     3549.3
Original Size (meg)     48289.8    31693.3    16596.6
Avg Compressed Size (%)    22.5       23.1       21.4   (level:#disks ...)
Filesystems Dumped           53         12         41   (1:41)
Avg Dump Rate (k/s)       812.7      952.5      585.1

Tape Time (hrs:min)        0:13       0:08       0:05
Tape Size (meg)         12963.5     9414.2     3549.3
Tape Used (%)               6.7        4.9        1.8   (level:#disks ...)
Filesystems Taped            53         12         41   (1:41)
Avg Tp Write Rate (k/s) 17549.5    20734.5    12469.2

USAGE BY TAPE:
Label               Time      Size      %    Nb
DailySet1-018       0:13    12963M    6.7    53

-----------8<-----------------------------------------------------------------

I'm still under 20Mb/Sec

Now, I figure the problem is that when I did the "dd" test, all my holding disk had to do is read the files. Now, it has to read the files whil amanda is pounding at it, wrinting dump files coming from the clients... and unfortunately, it seems my single SATA disk can't keep up with all those I/Os

What are my options ? We can't afford to buy a beefy server just to do backups. We are uning a centos beige commodity box. Even then, the server seems to be I/O bound. What could I do anyway ? Buy a faster disk subsystem ? What kind of subsystem ?

Of course, I could remove the tape from the drive, and in the morning manually run amflush, but it's not very very convenient or elegant and it will monopolize the tape for a couple of hours in case I need to restore something in an emergency it will be a mess.

Thanks