Systemd trying to get in...

The_Great_Sephiroth · Posted: Mon Jun 01, 2015 1:56 pm Post subject: Systemd trying to get in...

I have a clean install of Gentoo 64bit with systemd masked and "-systemd" in my global use flags. I also mask "sys-fs/udev". Now it wants to change to eudev, but it claims that there is a circular dependency and the only fix is to change virtual/libudev-215-r1 to use the systemd flag. What the heck? I am using the default profile, not a KDE/Gnome/whatever profile. The system is shell-only and is using btrfs in RAID1 for a file-server. Gentoo lives on an old, 40GB IDE disk separate from the array. So why is libudev trying to depend on systemd?
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charles17 · Advocate Joined: 02 Mar 2008 Posts: 3664

Hm, according to

The_Great_Sephiroth · Posted: Mon Jun 01, 2015 2:33 pm Post subject:

I figured it out. Portage was ruined. When I installed the OS my disk was sda, but it was now sdc in the actual OS. I guess I need to use UUIDs, but they're hard to write down and then type in. I'll just use UUIDs from now on. Thanks for the help, but this one is on me.
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depontius · Advocate Joined: 05 May 2004 Posts: 3509

The_Great_Sephiroth · Posted: Wed Jun 03, 2015 12:57 pm Post subject:

I am using BTRFS due to bit-rot protection. I am duplicating both the checksum data and the physical data. The only other system that protects this way is XFS, and it is a resource-hog.

Also, reinstalling and using UUIDs to mount fixed the issue. For some reason my kernel saw things differently than the live CD last time. Lesson learned, use UUIDs!
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depontius · Advocate Joined: 05 May 2004 Posts: 3509

You can do that with mdadm as well, you just have to put a scan into a cron job and let it run in the background.

White reminds me... I haven't done that yet, thanks for the jog.
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The_Great_Sephiroth · Posted: Wed Jun 03, 2015 3:31 pm Post subject:

I have never seen a thing about mdadm protecting against bit-rot. I would like to see some references to that. How does mdadm store checksums and such? Where? What RAID levels does it work with? At that point, since mdadm sets up the array, what file-systems does this work on? How would mdadm protect against bit-rot if I used exFAT or NTFS? Seems unlikely to me.
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depontius · Advocate Joined: 05 May 2004 Posts: 3509

Hard disks store checksums, that's not the issue with mdadm. The problem is that mdadm RAID-1 reads from the fast/easy drive, and if the read succeeds, doesn't question the other drive. It's possible for the second drive to have an error, and nobody knows.

I'm not sure why filesystems are starting to store checksums, hard disk drives do that. Are the hardware checksums for some reason insufficient?
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John R. Graham · Posted: Wed Jun 03, 2015 4:40 pm Post subject:

ZFS and BTRFS store cryptographic hashes instead of mere CRCs. They are (marginally) more reliable. Some see the benefit in some use cases (e.g., long-term archival storage).

- John
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The_Great_Sephiroth · Posted: Wed Jun 03, 2015 5:05 pm Post subject:

As far as I understand it, hard-drive checksums do not protect against bit-rot. Heck, they may not be able to. A single drive, even if it detected an error, could not recover the lost data. ZFS/BTRFS can due to the multiple disks. I am looking to preserve data for ages. I wish BTRFS would clone the checksums and such to more than two disks in RAID1. I figure a BTRFS RAID1 on four disks would be insanely secure.
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John R. Graham · Posted: Wed Jun 03, 2015 5:26 pm Post subject:

The trick pony that ZFS and BTRFS can provide is error detection above the physical layer intelligently integrated with traditional RAID. An undetected physical layer read error will be detected by the mismatch of the cryptographic hash, at which point the traditional RAID mechanisms can transparently correct the error and re-write the bad physical block.

- John
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The_Great_Sephiroth · Posted: Wed Jun 03, 2015 6:20 pm Post subject:

Which makes it more valuable to me than mdadm with ext4 on top. I hope the BTRFS team gets the rest done quickly, such as cloning everything on all RAID1 disks. It looks promising to me.
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depontius · Advocate Joined: 05 May 2004 Posts: 3509

I don't know if you read my whole post when I jumped in on this.

If you're going to use btrfs, make darned sure you've got a good UPS, and have it properly set up to shut down your system. As I mentioned, I didn't just lose data, I lost the entire btrfs filesystem. After the crash, "btrfs-scan" couldn't even find it.

I've had UPS setups in the past, but as of now both have died, and I've been wanting to get another, only this time I want to get a good one. I want one that can monitor my battery life, and one with electronics that won't crap out on me.

One expects to lose open files on a crash or power event without a UPS. One doesn't expect to lose the entire filesystem, beyond recovery. I've never had more than the expected crash issues with ext-N or xfs, only with btrfs. Nor have I ever had the mount command hang, requiring a reboot, except with btrfs. Maybe someday it will be ready.
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The_Great_Sephiroth · Posted: Fri Jun 05, 2015 2:12 pm Post subject:

It is working fine for us, so far. I have unplugged the running PC while files are being used on the BTRFS partition and still no issue. I will do some more testing but it seems OK to me.
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depontius · Advocate Joined: 05 May 2004 Posts: 3509

Things are generally OK until they're not. I'm just glad I had a full backup less than 12 hours old, at the time.
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The_Great_Sephiroth · Posted: Fri Jun 05, 2015 3:36 pm Post subject:

I have two external drives hooked up, so we have backups. If BTRFS gives me trouble I will determine whether or not it is worth using.
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depontius · Advocate Joined: 05 May 2004 Posts: 3509

Next attempted systemd encroachment - actually it's a gnome encroachment, but that's effectively the same thing.

On the update check I just did, icedtea-bin-7.2.5.3 wants to upgrade to icedtea-bin-5.2.5.5, but that requires gsettings-desktop-schemas-3.14.2, which of course brings us into the systemd era.

I looked at the icedtea-bin ebuild, and see the following line:

steveL · Posted: Sun Jun 28, 2015 5:32 pm Post subject:

NeddySeagoon · Posted: Sun Jun 28, 2015 5:52 pm Post subject:

steveL,

PARTUUIDS on MSDOS disklabels need to be used with care. Take for example

depontius · Advocate Joined: 05 May 2004 Posts: 3509

steveL · Posted: Mon Jun 29, 2015 1:35 am Post subject:

Ah OK, thanks jrg; so they're not quite as robust if you use the old-style MS-DOS disks.

Seems ok if you have a fixed partition scheme, and lvm on top of it, as you only shove space around via the device-mapper.

I'll stick with good old /dev/sda and motherboard chipset-drivers in the kernel.

IDK what it is, but I find that positively luxuriant compared to the old days of bindists with thousands of modules.

The_Great_Sephiroth · Posted: Mon Jun 29, 2015 1:45 pm Post subject:

I had not heard of PARTUUIDs before this thread. I only use GPT unless the system is so old it REQUIRES DOS partition tables. Did you know that some old boards even REQUIRE the partition to start on sector 63? If you put an advanced format disk in one of those systems you'd be hosed.

I will look into PARTUUIDs for my GPT system. Thanks for the info.
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steveL · Posted: Mon Jun 29, 2015 6:44 pm Post subject:

Oops for some reason I though neddy was jrg; sorry about that.

GS: good one; informing you of the option is all I was after.

NeddySeagoon · Posted: Mon Jun 29, 2015 6:48 pm Post subject:

The_Great_Sephiroth,

Even in the dim and distant past, where it was worthwhile to edit the partition table by hand, to recover some of the "wasted" space, I've never come across a board that required the first partition to start at CHS 0,1,0.

Monterboards have no idea of the partition table on the disc, they all load CHS 0.0.0 into RAM and some broken BIOSes check for the signature word at the end of the partition table and the bootable flag on ore or more of the primary partitions in the position where the MSDOS partition table would be.

GTP provides all of there features except setting the bootable flag on the 'protective' MSDOS partition table it provides. You need to use

John R. Graham · Posted: Mon Jun 29, 2015 6:55 pm Post subject: