Seeking explanation for default mount option (solved)

causality · Apprentice Joined: 03 Jun 2006 Posts: 239

I recently purchased an 8TB external USB3 disk drive (Seagate model STEL8000100). Its primary purpose is for backups so I want to fully understand every option applied to it. This is an Advanced Format device (and also an SMR disk). Its physical sector size is 4k, though it reports a logical sector size of 512b to the BIOS (I think Linux is smart enough to just use the 4k figure).

I nuked the NTFS partition it came with and used the utility gdisk to create a GPT partition. This partition is aligned correctly at the first sector (that is, the program parted's align-check function says "yes" for "min", though "no" for "opt" - I assume because the partition takes up the whole device, which does not end on a sector number evenly divisible by both 4k and 512b - that's OK for me). Please advise if my assumption there is wrong.

I then formatted that partition using mkfs.ext4, at which time I specified a 4k block size on the command line. All other options are defaults as specified in the (unmodified) Gentoo-supplied /etc/mke2fs.conf file. That files does not set a "stripe=" mount option anywhere, nor did I manually add one.

Anytime I mount it specifying no options (i.e. mount /dev/sde1 /mountpoint), the mount option "stripe=8191" is automatically appended. Apparently this was automatically added as a default mount option as shown by running "tune2fs -l /dev/sde1".

I have always associated "stripe" and its related options with RAID arrays. Every Startpage search I make results in RAID information, which makes sense. However, this is not a RAID array. It is a single hard disk, containing a single partition, hosting a single filesystem. I'm not even using LVM or anything. It's just a plain disk.

I can't find any good reason for this. Why was that option appended and where did it come from? Is it important? Will there be some problem if I remove it from the default mount options? Is it helping anything? Considering that these are all open source tools with generally excellent documentation, it's astonishingly hard to answer these questions.

As this is a backup device I would feel better completely understanding it. Mysterious options I did not set, cannot trace, and do not seem to fit my use case, well that concerns me a bit.

Please advise.

Ant P. · Watchman Joined: 18 Apr 2009 Posts: 6920

See e2fsprogs/misc/mke2fs.c:1423 - it looks up the right write size from /sys/block/sd?/queue/{minimum,optimal}_io_size and sets the stripe width in the superblock from that. The kernel just picks up the option from there. It's labelled "raid_stripe_width" in the code but that's mostly for historical reasons at this point.

causality · Apprentice Joined: 03 Jun 2006 Posts: 239

I was actually just looking through the source when I received the e-mail notification of your reply.

What threw me off is that none of my other drives have this default mount option set in the superblock. If they had, I would have regarded it as normal behavior. I suppose this is because my other filesystems were all created years ago and in the past, mke2fs didn't behave this way unless you explicitly used the -E stripe=X option at filesystem creation time. I'm not sure about that (never considered RAID options before at all) but it would make sense.

So far as I know, I can use tune2fs to remove this option with no harm, since I have no RAID array.

Regarding alignment, this is where the drive stands right now according to parted:

Sadako · Posted: Mon Apr 03, 2017 7:38 pm Post subject:

One question, why partition the disk at all if you're only going to have one partition take up the entire disk anyway?

Only reason I can think of is if you're booting from that device, which doesn't seem to be the case here.
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causality · Apprentice Joined: 03 Jun 2006 Posts: 239

Sadako: partly out of habit and partly because I don't know if "mkfs.ext4 -b 4096 /dev/sde" would result in a favorable sector alignment. Do you know for certain if it would?

The downside of my choice to partition: according to gdisk sector number 34 is the "first usable sector". 4k sectors, first usable sector is 34, filesystem starts on sector 2048. That's 4096 * (2048 - 34) = 8249344 bytes unused. On an 8TB drive, this is such a tiny fraction of one percent that I consider it negligible. For that tiny (practically zero) price, I gain the certainty of avoiding the performance hit of all the excess I/O caused by misalignment.

This drive isn't hosting a database or anything like that where the absolute max performance is critical (I'd shell out for an SSD in that case). In fact my favored trade-off in this instance was: a larger drive with a low price-per-GB rather than spending the same money for a smaller but faster disk. Indeed, inside the USB3 enclosure is a Seagate Archival drive. Still, I intend to store large amounts of data so I'll take whatever performance gain I can get especially when said gain is essentially free.

I think I'm satisfied with my setup now, but still I wonder: do you know of any advantages to just using the entire /dev/sde block device without partitions? Any disadvantages? Am I overlooking something potentially significant? The great thing about a forum like this, is that one can learn much just by listening.

Sadako · Posted: Mon Apr 03, 2017 8:28 pm Post subject:

Yes, "mkfs.ext4 -b 4096 /dev/sde" would result in optimal alignment, think of it this way, when partioned you need to concern yourself with both the partitioning scheme AND the filesystem wrt alignment, using the whole disk you eliminate the former.

That said, there's nothing "wrong" with a single partition taking up the whole disk, only issue is wasting of a few bytes which is absolutely inconsequential giving the size of the disk (although, iirc gpt creates an identical partition table at the end as well as the start of the disk).
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causality · Apprentice Joined: 03 Jun 2006 Posts: 239

There is one more thing I don't understand: how to get rid of that stripe= mount option.

I unmounted the device. I ran tune2fs -E mount_opts="" /dev/sde1. I also ran "tune2fs -o ^acl,^user_xattr". Currently this is the output of tune2fs /dev/sde1:

Sadako · Posted: Mon Apr 03, 2017 9:09 pm Post subject:

wrt relatime, from mount man page;

causality · Apprentice Joined: 03 Jun 2006 Posts: 239

Since I never specified that, nor any other RAID related parameter, I assume that even repartitioning and reformatting from scratch would yield the same result.

I was finally able to get rid of it via "tune2fs -E "" /dev/sde1". Prior to that I tried your command with the stripe= syntax. One of those got it. I may make a feature request to upstream e2fsprogs, asking that the tune2fs program do a better job of showing currently set extended mount options. That would have saved me some time trying to track this down.

I really appreciate all the help with this, especially considering it's a relatively minor issue. Sometimes I want to solve a puzzle just because I feel like I should be able to understand it, and because it's there

. It's why a lot of mountaineers climb mountains. They learn much along the way.

Nobody beats the Gentoo user community in terms of expertise and willingness to share knowledge. It's a major reason I've stuck with Gentoo all these years. Well, that and I truly love what Gentoo is all about (choice) and the way they do things. In fact this forum is one of the first places I look for any Linux question, even if it's another distribution, just because the information is so good.

shrike · Posted: Tue Apr 04, 2017 1:14 pm Post subject: