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I recently got an external SAS LTO-5 tape drive from eBay for my NAS at home, and a SAS PCI-E adapter - this is the sas2flash output:

LSI Corporation SAS2 Flash Utility
Version 20.00.00.00 (2014.09.18) 
Copyright (c) 2008-2014 LSI Corporation. All rights reserved 

        Adapter Selected is a LSI SAS: SAS2008(B1)   

Num   Ctlr            FW Ver        NVDATA        x86-BIOS         PCI Addr
----------------------------------------------------------------------------

0  SAS2008(B1)     20.00.07.00    14.01.00.06    07.01.00.00     00:01:00:00

        Finished Processing Commands Successfully.
        Exiting SAS2Flash.

I had to flash a new firmware on the card (it had a "fault state" message on dmesg, but now it's fixed) and had to patch the LTFS userspace tools to get them to recognize this model, since it's made by IBM but a Dell rebrand. The patch is here.

All is good, but I'm getting poor performance from it. Usually, reads from my RAID5 array are in the 70-110MB/s range over Samba to the rest of my home network, but I can only get the tape drive connected to it via the SAS port on the adapter card to do ~22 MB/s, which makes filling up a 1.5TB tape a lengthy ordeal.

Is there any way to debug where the problem may lie? The NAS is on a mini-ITX motherboard and rocks 8GB of ram and a 4x3TB RAID5 array, plus a boot SSD connected to one of the adapter card's internal SATA ports. The SFF-8088 cable is 1.5m long, but the drive just currently stands on top of the NAS case.

Any help appreciated!

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  • What software are you using to write to the tape? I found a lot of software that couldn't even keep an LTO3 drive busy. Mar 10 '17 at 9:52
  • I'm just using LTFS, haven't tried using tar yet. Mar 10 '17 at 12:05
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I got three theories.

  1. Drive failure. Similar to HDDs, tape drive has many types of mechanical components such as actuator, motors and rollers etc. If these components fail, tape drive retries write process many times until success, which results in slow data transfer. Also, debris on the head has a negative impact on the performance. Since you bought the drive from eBay, the drive condition can be degraded. I would recommend to load a cleaning cartridge called UCC to make sure that head is clean.

  2. Media failure. Similar to the drive failure, tape can also be damaged due to drop, vibration and excessive usage in the drive. You can easily isolate this issue by using a brand new tape.

  1. LTFS is not good at writing a bunch of small files. The reason is because LTFS needs to write meta data on the tape as well as user data. If file size is large enough (e.g., GB), this over-head of meta data can be negligible. However, if the file size is small (e.g., kB) and comparable to meta data size, transfer rate will be slow.

I would also recommend IBM Tape Diagnostic Tool (ITDT) to isolate the cause. For example, ITDT has a several types of diagnostic tests such as "Health Test", "System Test" and "Full Write". If these tests fails, the cause is your drive or media - not LTFS.

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