Western Digital My Book Duo DAS Review
by Ganesh T S on July 12, 2014 6:00 PM EST- Posted in
- Storage
- Western Digital
- DAS
Miscellaneous Aspects and Concluding Remarks
Some of the miscellaneous aspects of interest in the WD My Book Duo include RAID rebuild and power consumption. The LEDs in front begin to flash red when the volume enters a degraded state (and also during the rebuild process). The WD Drive Utilities provides insights into the health of the array. We simulated drive loss by pulling out one of the disks during data transfer (the array was obviously configured in RAID 1). The hardware LED status immediately began flashing red. The monitoring program also reflected the degradation, but, only after the drive had been dismounted and remounted. Inserting the removed disk into a PC's SATA slot didn't show the stored data (as expected, since this is hardware RAID). The gallery below provides more insight into our RAID rebuild evaluation.
The unit does not support hot-swapping drives (unlike the 2big Thunderbolt 2). After dismounting, a new WD Red drive of the same capacity was inserted. At this juncture, WD provides us two ways to rebuild RAID - either by pressing the reset button on the back of the unit, or, via the WD Drive Utilities software interface. We took the latter approach, as evidenced in the above gallery.
Coming to the business end of the review, the WD My Book Duo is a very attractive solution for users looking to get high speed access to large amounts of data. Advertised speeds are reached for certain types of workloads. USB 3.0 support is now almost universal, and eSATA is losing favour in the mass market. Therefore, the choice of a USB 3.0-only interface for this RAID-enabled DAS solution looks perfect. The inclusion of a two additional USB 3.0 ports in a hub configuration enhances the versatility of the unit. It makes sure that average users (who are not focused on getting maximum bandwidth to their peripherals all the time) don't need to 'sacrifice' a USB 3.0 port in their systems. There is also a comprehensive software suite to take advantage of the high-speed DAS.
There are really no negatives to talk about with respect to the My Book Duo. While earlier DAS units from Western Digital allowed only certain drives to be used for RAID rebuilds (by including a firmware check before allowing the rebuild), the My Book Duo has no such problems. Western Digital confirmed that the Red and Green drives are certified for rebuilds, but there are no restrictions in place regarding usage of any other SATA drive model of the appropriate capacity. Potential areas of improvement, however, include support for hot-swapping drives and provision for data recovery from a RAID 1-member drive directly connected to a PC.
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voicequal - Saturday, July 12, 2014 - link
Why isn't the RAID 1 read performance closer to the RAID 0 read? Can't data be read from both drives in RAID 1?PEJUman - Saturday, July 12, 2014 - link
While in general I agree with your sentiment, I thought about this question before and one possible answer I came up with was to save the wear and tear on the 2nd drive. i.e. it only uses the 2nd drive when the 1st one have too much ECC.This approach matches well with the raid 1 goal of ultimate redundancy.
Ultimately, I wish more controller would expose the finer details on Raid tuning such as this option
madmilk - Sunday, July 13, 2014 - link
Not for sequential reads, because RAID 1 isn't striped. On RAID 0 you can read alternating stripes from each drive sequentially, but with RAID 1 you'd be reading the data twice.The random read scores are much closer between the two.
voicequal - Sunday, July 13, 2014 - link
I see your point that the reads won't be 100% sequential as seen by the drive heads, but if drive 1 starts reading at X and drive 2 at X+128KB, you can effectively get twice the read throughput over 256KB. Then you have to move the drive heads +128KB which does incur a performance cost.Still with a sufficiently large read block size, I would think there could be a substantial performance improvement reading from both drives in RAID 1. Does anyone know a RAID1 HW or SW controller that can do this?
DanNeely - Sunday, July 13, 2014 - link
The time spent skipping ahead is equal to the the time spend reading the area being skipped in a non-fragmented file. To double read speeds in a "mirrored" drive you'd need to have either the array controller or the driver in a software array store the file sectors as 02481357... on the first drive and 13570248... so that when reading the file the two drives are reading sequential sectors on the drive and alternating chunks of data in the file.Cerb - Sunday, July 13, 2014 - link
No, you wouldn't. You'd just need to alternate drives for reads, keeping them balanced, so that a total QD of say, 6 would be QD=2-4 on one drive, and QD=2-4 on the other. Where the file data actually gets stored shouldn't matter, only how the RAID implementation decides to read it. If the reads are sufficiently sequential, both drives should be able to stay quite busy, and get read performance around that of RAID 0.Most likely is that they didn't bother even trying that, as RAID 1 is not generally used for performance anyway.
voicequal - Sunday, July 13, 2014 - link
Your approach would make sequential reads quite fast, but at the expense of sequential writes which would be split across different areas of the drive.xfortis - Sunday, July 13, 2014 - link
This is a good question. I assume that most drives are set up in their controllers to present data sequentially from the beginning. I don't think it's very common that any type of program would ask a storage device for the second-half of a given file (at least not without having read the first half); I would think that the drive wouldn't have the capability within itself to address data beginning at an arbitrary point in a sequence of data - it always has to start at the beginning of the data(?).I think to implement this you would need to segment your data at the storage/RAID controller level, like striping but each drive has all the stripes in a RAID 1. Then at the controller level the controller would be able to take a request for data and, assuming the requested data spans at least two segments, it can produce two or more starting-addresses for the drives to read. But then your segment-size would have to be tuned to the kind of data you have (like allocation units) and also then there would be an additional level of addressing abstraction/complexity that would make any kind of data-recovery very difficult.
Everything I just said may be wrong. I'm just making assumptions and inferences because it's fun. Let's get a volunteer who has more knowledge or feels like trawling wikipedia for a while!
voicequal - Sunday, July 13, 2014 - link
Yes, I'm thinking this would be best done at the controller level. I've seen operating systems apply their own striping of sorts at the filesystem (i.e. NTFS) level. Try writing two large files simultaneously to the same hard drive. On an OS like Windows 8, the throughput is surprisingly good. This can only be achieved if the OS is smart enough to use a reasonably large "chunk" size for writing the file fragments to the disk. In this way the disk sees mostly sequential write activity despite the two concurrent write operations, while the number of file fragments tracked by the filesystem is minimized.TerdFerguson - Saturday, July 12, 2014 - link
If it can't connect directly to a router and it can't host a Plex server, I'm not interested.