RAID5 SSD: Pros & Cons of Replacing Failed HDD w/SSD

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Discussion Overview

The discussion revolves around the considerations of replacing a failed HDD in a RAID5 configuration with an SSD, particularly in the context of a homebrew DVR system. Participants explore the implications of using SSDs versus HDDs, including reliability, cost, performance, and the specific use case of DVR recording and playback.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes the failure of a Seagate HDD and the potential to replace it with an SSD, noting the cost and reliability of SSDs compared to HDDs.
  • Another participant argues that RAID performance will be limited by the slowest drive, suggesting that a hybrid SSD/HDD RAID may not yield the expected speed benefits.
  • Concerns are raised about the impact of filling SSDs to capacity, which could lead to premature failure, and how this might affect RAID configurations.
  • Some participants discuss the specific use case of DVRs, suggesting that the recording and playback patterns may mitigate speed concerns and that large files dominate the data storage.
  • There is a debate about the reliability of data recovery from HDDs versus SSDs, with some suggesting that HDDs may offer better recovery options in certain failure scenarios.
  • One participant mentions the potential for a mixed SAS/SATA system, comparing it to the SSD/HDD hybrid approach, while also considering noise levels in a DVR context.

Areas of Agreement / Disagreement

Participants express a range of views on the feasibility and implications of using SSDs in a RAID5 setup, with no clear consensus on the best approach. Some agree that SSDs could work well in this context, while others caution against potential performance limitations and reliability issues.

Contextual Notes

Participants acknowledge various limitations, including the potential for SSDs to fail when filled to capacity and the complexities of RAID performance with mixed drive types. The discussion also highlights the specific recording patterns of DVR systems, which may influence the effectiveness of different storage solutions.

  • #31
pbuk said:
Yes I find some of those numbers a bit odd too -
@pbuk, I looked at the numbers more carefully and actually <shudder> looked at the documentation.

Pure mirrors and stripes behave as expected. Parity RAID sometimes looks wacky. One thing I learned is that the RAID5 implementation cheats. If you have X bytes to store, you would expect it to put X/2 on one drive, X/2 on another, and have a X/2 parity block on a third. However, there is a minimum block size. If X is less than the minimum block size, the system mirrors the data on 2 of the 3 drives and there is no parity block.

The storage utilization is still inefficient, just not as inefficient as it could have been: 2 partially filled blocks rather than 3. But it means that performance will be somewhere between a true RAID5 and a mirror.

The next complication is that these are likely 4Kn/512e drives operating with a 512 sector size.

The data is uncompressed, and in real life it would be. This is a 20-30% effect on most of my data, and it some cases surprisingly good: my backups, which Windows itself compresses, is shrunk by 13%. That means 13% higher throughput. On user data it's even better: 36%. Since parith compresses less well tnan data, this is an additional penalty for paity RAID.

So, while I don't quantitatively understand the parity RAID performance, I am less surprised now.

pbuk said:
I'm particularly surprised the only data for 4 disks is for RAID 6 and RAID 10, surely RAID 5 is the optimal compromise with 4 disks for most situations.

I'd use RAID5 for three. and do. But 4? I'd set it up as a pair of mirrors. Yes, I get only 2/3 of the capacity, but I'll get faster reads, faster writes, a good chance of surviving a double failure, and less intensive/risky rebuilds. Further, if I need more capacity, I can keep the pool up and add only two disks at a time, not all 4.
 

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