Slow write speeds


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Hello. Having running my unRaid server for several months, I must say I`m very satisfied with the overall system. The only thing that`s still bothering me is the network write/read speed. Having 5 disks in the array (6tb parity + 4x6tb data), average speed on write tends to be 35-65 Mbit/s. My disk setup has XFS as default file system .

Specs:

- 4x WD RED + 1x Seagate Archive

- 1 gb network card + 1 gb switch 

- 8 gm RAM

- i3 CPU

- ALL HDDs RUN ON SATA (MBs chipset)

 

I`d like to raise some questions:

1) Can I somehow speed up my network write/read speed? (I mean copying data via SAMBA over 1gb ethernet switch)

2) Is my array set in some sort of RAID or does XFS work more like JBOD?

 

Thax for your help :)

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Normal writes are slow because the system performs a read/modify/write operation - to be able to update the parity it needs to know the old contents of the data disk before it overwrites with new content. But the system only needs to have the parity drive(s) and the single data disk spinning.

 

If you enable turbo write, then system will need to have all disks spinning. But will all disks spinning it can compute new parity by reading from the other disks so every single disk will either perform a read access or a write access.

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To tuen on 'Turbo write'    Settings   >>>   Disk Settings   and set   'Tunable (md_write_method):'  to " reconstuct write".

 

However, be aware that writing a lot of small files to the array is always much slower (because of file creation overhead and write head movement) than for  large files.  IF you want max write speed from a user perspective, then use an SSD cache drive. 

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When lots of small files are copied, then a SSD cache may not be enough, because the HDD source disk will also suffer from the large number of seeks to locate all the small data blocks. From the perspective of the source disk, copying small files is similar to copying heavily fragmented files.

 

Edit: Forgot to mention that if copying small files, then the network overhead of SMB will also play a big role.

Edited by pwm
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On 5/8/2018 at 7:03 PM, unP said:

2) Is my array set in some sort of RAID or does XFS work more like JBOD?

 

There is "some sort of RAID" in the sense that redundancy is provided by the parity disk(s). Unlike most RAID levels data isn't striped across multiple physical disks. Each disk has its own file system (so there is a JBOD element) but user shares allow file systems on different disks to be aggregated together. Hence the name: unRAID.

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1 hour ago, John_M said:

 

There is "some sort of RAID" in the sense that redundancy is provided by the parity disk(s). Unlike most RAID levels data isn't striped across multiple physical disks. Each disk has its own file system (so there is a JBOD element) but user shares allow file systems on different disks to be aggregated together. Hence the name: unRAID.

 

That was exactly what I meant ! 

 

Thx for all replies guys :)

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The redundancy is the same as a RAID - same concept with parity computed from multiple data disks.

 

You gain the advantage that if more disks dies than you have parity, the remaining data disks will still be usable since the information isn't striped - a traditional RAID breaks down completely if they suffer one-too-many disks failures.

 

The disadvantage is that you don't gain the additional bandwidth from striped access.

 

If the goal is huge bandwidth, then a traditional RAID will win. If a large percent of your data is media files, then you gain the huge advantage that you can stream a media file from one data disk while the other disks are sleeping. So less noise and less electrical bill.

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