Re: [gentoo-user] Seagate hard drives with dual actuators.

[Michael <confabulate@kintzios.com>]

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On Wednesday 13 November 2024 23:10:10 GMT Dale wrote:
> Howdy,
> 
> One of my PVs is about 83% full.  Time to add more space, soon anyway.
> I try not to go past 90%.  Anyway, I was looking at hard drives and
> noticed something new.  I think I saw one a while back but didn't look
> into it at the time.  I'm looking at 18TB drives, right now.  Some new
> Seagate drives have dual actuators.  Basically, they have two sets of
> heads.  In theory, if circumstances are right, it could read data twice
> as fast.  Of course, most of the time that won't be the case but it can
> happen often enough to make it get data a little faster.  Even a 25% or
> 30% increase gives Seagate something to brag about.  Another sales tool.
>  Some heavy data users wouldn't mind either.
> 
> My question is this.  Given they cost about $20 more, from what I've
> found anyway, is it worth it?  Is there a downside to this new set of
> heads being added?  I'm thinking a higher failure rate, more risk to
> data or something like that.  I think this is a fairly new thing, last
> couple years or so maybe.  We all know how some new things don't work out.
> 
> Just looking for thoughts and opinions, facts if someone has some.
> Failure rate compared to single actuator drives if there is such data.
> My searched didn't help me find anything useful.
> 
> Thanks.
> 
> Dale
> 
> :-)  :-)

I don't know much about these drives beyond what the OEM claims.  From what I 
read, I can surmise the following hypotheses:

These drives draw more power from your PSU and although they are filled with 
helium to mitigate against higher power/heat, they will require better cooling 
at the margin than a conventional drive.

Your system will use dev-libs/libaio to read the whole disk as a single SATA 
drive (a SAS port will read it as two separate LUNs).  The first 50% of LBAs 
will be accessed by the first head and the last 50% by the other head.  So 
far, so good.

Theoretically, I suspect this creates a higher probability of failure.  In the 
hypothetical scenario of a large sequential write where both heads are writing 
data of a single file, then both heads must succeed in their write operation.  
The cumulative probability of success of head A + head B is calculated as 
P(A⋂B).  As an example, if say the probability of a successful write of each 
head is 80%, the cumulative probability of both heads succeeding is only 64%:

0.8 * 0.8 = 0.64

As long as I didn't make any glaring errors, this simplistic thought 
experiment assumes all else being equal with a conventional single head drive, 
but it never is.  The reliability of a conventional non-helium filled drive 
may be lower to start with.  Seagate claim their Exos 2 reliability is 
comparable to other enterprise-grade hard drives, but I don't have any real 
world experience to share here.  I expect by the time enough reliability 
statistics are available, the OEMs would have moved on to different drive 
technologies.

When considering buying this drive you could look at the market segment needs 
and use cases Seagate/WD could have tried to address by developing and 
marketing this technology.  These drives are for cloud storage 
implementations, where higher IOPS, data density and speed of read/write is 
desired, while everything is RAID'ed and backed up.  The trade off is power 
usage and heat.

Personally, I tend to buy n-1 versions of storage solutions, for the following 
reasons:

1. Price per GB is cheaper.
2. Any bad news and rumours about novel failing technologies or unsuitable 
implementations (e.g. unmarked SMRs being used in NAS) tend to spread far and 
wide over time.
3. High volume sellers start offering discounts for older models.

However, I don't have a need to store the amount of data you do.  Most of my 
drives stay empty.  Here's a 4TB spinning disk with 3 OS and 9 partitions:

~ # gdisk -l /dev/sda | grep TiB
Disk /dev/sda: 7814037168 sectors, 3.6 TiB
Total free space is 6986885052 sectors (3.3 TiB)

HTH

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