## raid – Dell R720 – can’t install any operating system

I am trying to install a host OS but whatever I try installing, it always fails with I/O related error. Looking at similar issues described on the internet, everything points to an imminent / existing disk failure. I have tried multiple RAID combinations to narrow down the issue but no luck so far. The machine is a Dell R720 with PERC H710p RAID controller and it originally came with 6 x 600GB 6G SAS 10k 2.5″ drives. Tried these:

• 1 disk RAID 0 (group 1), 5 disks in RAID 5 (group two): errors when attempting install on /dev/sda (group 1) and /dev/sdb (group 2), tried using different disks from the bay to form same groups
• 3x RAID 1: errors when attempting install on /dev/sda, /dev/sdb and /dev/sdc
• removed 3 drives, tried 1x RAID 5 on all 3 remaining drives: errors in the same way

The operating systems I tried so far:

• Alpine 3.13: reported I/O error, installer exits to ash when trying to write the partition table
• Ubuntu 16 LTS: reported I/O error right at the start of the installation process
• Ubuntu 18 LTS: udevadm settle retried multiple times, I/O errors reported right before, installer crashes and restarts to region selection
• Ubuntu 20 LTS: same as Ubuntu 18
• CentOS 7: reported python anaconda error when trying to write to disk before I was able to even put the root password in, installer hangs, machine requires hard reboot
• XenServer 7.0: installer stopped at 68%, machine required hard reboot

With every one of these, regardless of which disk group (VD?) I use for the OS, as soon as the installer attempts writing the partition table, all disks from the selected disk group start blinking amber. Ubuntu 18 / 20 consistently when it is time to put the user name, server name and password. After reboot, disks are blinking green again. In RAID configuration (CTRL+R), all disks are online, VD state is reported as Optimal. I have SATA AHCI set in boot properties in BIOS.

I ran the lifecycle manager tests on the server, everything is dandy. No errors reported, except of the missing PERC battery as the server does not have one physically installed. I understand why I would need this battery for data consistency on power loss but it should not prevent me from installing the OS? I suspect that the RAID controller is faulty but I am not an expert.

Is there anything else I can do to further diagnose the problem?

## ordinary differential equations – Can the fundamental system of solutions consists of $n+1$ functions?

Thanks for contributing an answer to Mathematics Stack Exchange!

But avoid

• Making statements based on opinion; back them up with references or personal experience.

Use MathJax to format equations. MathJax reference.

## How To Develop A Pharmacy Management System

Are you planning to set up a new pharmacy or intend to revamp the present one? Well, going by the fact that technology is blooming and the medical sector is crucial sector in a community, the software development of this sector is quite crucial, isn’t it? Let’s dive in the custom pharmacy management software development

## usability – Visibility of system status for long term status

The first heuristic of Nielsen speaks about “** Visibility of system status **” that suggests the designer keep every information the user about the state of some operations, the classic example is the upload percentage made visible with the upload bar.

This rule is valid also for not real-time operations? For example, if I send a mail to the customer service, the status of this request is to be considered a “Visibility of system status”?

e.g in my profile page I could have a box with this sentence:

Your request is waiting for a response.

Other examples that I’m not sure to consider in that heuristic rule are: the red dot in a Facebook notification or, call into question StackExchange, user badges, progress, etc are handled with this rule?

If not how?

## computer architecture – Minimal number of resources such that the system is considered safe?

https://ibb.co/1d7cqXw

I understand that the minimum number of resources should be computed by the following formula:

R >= P * (N-1) + 1


meaning that we would have:

R >= 8 * (74+59+64+34+29+44+29+29) + 1
R >= 8 * 362 + 1
R >= 2897


However, this does not seem right. The number seems too big and I don’t see the purpose of the initial allocation in this exercise.

Am I doing something wrong? Am I using the wrong formula?

## Database consistency for a distributed system

I’m working on a phone call monitoring project. The aim is to have one row in the calls table for each ongoing call. Each call instance may be updated by many different servers/threads as more information is gathered about the call. Each call instance (a row) is mapped to a Java object. When a server/thread needs to update a call, it will read the call, apply the update, and then write the call back to the database.

This obviously introduces consistency problems, if a separate update U2 writes in between the read and write of another update U1, then when U1 is written, it writes stale data.

My current idea to fix this is to capture the changes for an update. When an update happens, it will read a “last updated” time. Then, the changes are applied and written back to the database conditionally (only written if last updated is equivalent to what was read). If the “last updated” time has changed, then a retry is executed, re-reading the data, applying changes, and attempting another write. This will retry the write 3-5 times before giving up.

This has some drawbacks in regards to performance, but is the best I was able to come up with. There is also the rare yet possible problem of retries executing out of order (across different updates). I’m hoping to make all changes independent of the order they are executed in, and I know that order shouldn’t be relied on. A good example of this is call status. Say update U1 sets a call status to ongoing and U2 sets it to finished. If U1 fails, U2 completes successfully, and then U1 completes on a retry, the status will be incorrectly set to ongoing. One option is only allow ongoing to be set when the call status isn’t finished, but this isn’t always clear of all fields and feels a bit messy. There are always options to try to update columns individually, come up with some sort of merging policy, or something else.

My primary goals are consistency and performance. While these criteria are somewhat contradictory, NoSQL seems like a decent tradeoff for my goal (my decision is described more below).

My questions:

• Is there a better way to achieve this goal? It feels sloppy to have every change get recorded as deltas and executed in a retry loop, but I’m not sure if there’s a better way to do it.
• I’m currently using NoSQL. Data loss is tolerable, so replication is disabled. I am using conditional transactions to implement updates. Is this the right technology for the job? With replication disabled, I expect that to help considerably in addressing the performance hits introduced by transactions. Scylla/Cassandra’s ring hashing seems to be an efficient way to distribute the data, especially with the added benefit of handling call data close to its geographic location. I’ve also considered Redis, but decided against it as the added database i/o isn’t significant, as well as Redis’s (somewhat?) poor distributed performance.

It seems like locks would work well in this scenario, but I’m not sure if NoSQL/Scylla supports locks. Should I switch to something that does?

Appreciate the help, will edit with any information if needed. Thanks!

## samsung galaxy s 9 – Using NetGuard, what app(s)/service(s) can be restricted to prevent system updates?

I’ve been using NetGuard for a little while now, and I’ve really liked it so far. I hate the forced system updates which seem to come more and more frequently. For a handful of reasons, I’d like to prevent this from happening via NetGuard, if possible. I have to imagine there’s a service or two which compares my Android version with the latest available version, and perhaps another which actually downloads the latest version. The problem is that the list of apps and services to restrict are vast, and I have no idea what most of them do. What on this list needs to be restricted in order to prevent all future forced updates?

## windows 10 – Can’t boot after cloning system to SSD

I’m trying to migrate a Windows install to a SSD. The current HDD contains installs for both Windows and Ubuntu, as well as data partitions.

The SSD is smaller than the HDD, so I copied each partition individually using the Linux’s dd tool, instead of coping the whole drive.

The HDD partitions were:

 -UEFI
-Windows system
-Windows recovery
-Linux system
-Linux swap
-Linux data
-Windows data


And the SSD:

 -UEFI
-Windows system
-Windows recovery
-Linux system
-Linux swap


The partitions are GPT on both drives.

Initially the system would boot using the old drive if it was connected and give a ‘device not connected’ blue screen error if it wasn’t. The Windows Boot Manager was still pointing to the old drive, so after using bcdedit to change the drive, the system starts to boot but gets stuck in a black screen with spinning dots.

I also noticed that in the recovery environment the HDD partitions were assigned earlier letters, so the old windows partition is C:, while the new one is F:. Swapping the sata connections put then in the ‘correct’ order, but didn’t solve the booting problem

The possible problems I have identified:
*The system is stuck on the black screen with spinning dots. After a while the monitors will turn of until I move the mouse or press a key, but otherwise there was no change even after a couple hours.
*When entering the recovery environment (by restarting three times), the recovery partition being used is the HDD one. If I attempt to start in the recovery environment with the HDD unconnected, I get the ‘device not connected’ blue screen. After changing the sata I always get the blue screen, but pressing f1 starts the recovery environment anyway if the HDD is connected.